Introduce rebar-version (#9)

Previously (Remove redundant code) (+27 squashed commits)
Squashed commits:
[0b79c31] Rename as setup-beam
[312b42b] Simplify platform verification
[b43ca96] Prevent use of OTP- prefixed Erlang/OTP versions
[103a236] Move the promise-based approach to an async-based approach

(not testing for failure, so not guaranteeing readable/usable output either)
[1e7f631] Remove local test files
[d03e0c6] Prevent use of -otp- infixed Elixir versions
[697110e] Have versions output in action, not just in tests
[7360781] Ease test validations
[71e6999] Reinstall tests that were there before
[6602f20] Attempt at simplifying regexp matching with build listings
[09247af] Provide for an Elixir to "not OTP" fallback mechanism

(as explained by the comments)
[dfc4083] Attempt at renaming for clarity

We also get rid of return values when not reachable
We also move the `-otp-` part of the Elixir version upstream so it's easy to reason about
  (and update the tests accordingly)
[0d4eb28] Bump our dep.s

(trying to figure out why core.setFailed isn't making the action fail, as it should)
[ebf48ef] Have action fail (with an exception) when no expected version is found

(we otherwise feel that a simple warning is not enough, as it may end up hiding potential bugs)
[8478364] Adapt tests to our actual inputs
[16ea63d] Add prettier again, now sync'ed with jslint

(pass it on the code, already)
[cfc65df] Improve error messages and tests
[a017948] Test unavailable Elixir upon mix
[7f2a3e9] Test failing installations

Also output core.error when expected
[08061ca] Attempt at making shell scripts more robust
[4ee73cb] Separate test code from production code

1. inputs (for CI) are managed by code, not by .yml env. variables
2. rebar3_builds.txt added for stable repeatability
3. tests tweaked to make it easier to modify and drop dependency on production code
4. `dist` regenerated (duplicate files gone)
5. setup-elixir.js tweaked to this new reality
[d17fed4] Remove prettier from the package
[7d6f8f5] Actually break CI when we should
[b972eaf] Remove unrequired element
[0ffa28a] Ease maintenance and readability
[4690169] Fix as per broken tests
[a1371c6] Merge setup-erlang in

README.md

@@ -1,27 +1,32 @@
-# setup-beam
+<!-- markdownlint-disable MD013 -->
+# setup-beam [![GitHub Actions Test][test-img]][test] [![GitHub Actions CI][ci-img]][ci]
 
-[![](https://github.com/erlef/setup-beam/workflows/Test/badge.svg)](https://github.com/erlef/setup-beam/actions)
-[![](https://github.com/erlef/setup-beam/workflows/Licensed/badge.svg)](https://github.com/erlef/setup-beam/actions)
+[test]: https://github.com/erlef/setup-beam
+[test-img]: https://github.com/erlef/setup-beam/workflows/test/badge.svg
+[ci]: https://github.com/erlef/setup-beam
+[ci-img]: https://github.com/erlef/setup-beam/workflows/ci/badge.svg
 
-This action sets up an Elixir environment for use in a GitHub Actions
+This action sets up an Erlang/OTP environment for use in a GitHub Actions
 workflow by:
 
-- Installing OTP
-- Installing Elixir
+- installing Erlang/OTP
+- optionally, installing Elixir
+- optionally, installing `rebar3`
 
-**Note** Currently, this action currently only supports Actions' `ubuntu-` runtimes.
+**Note** Currently, this action only supports Actions' `ubuntu-` runtimes.
 
 ## Usage
 
 See [action.yml](action.yml).
 
-**Note** The OTP release version specification is [relatively
+**Note** The Erlang/OTP release version specification is [relatively
 complex](http://erlang.org/doc/system_principles/versions.html#version-scheme).
-For best results, we recommend specifying exact OTP and Elixir versions.
+For best results, we recommend specifying exact Erlang/OTP, Elixir versions, and
+`rebar3` versions.
 However, values like `22.x` are also accepted, and we attempt to resolve them
 according to semantic versioning rules.
 
-Additionally, it is recommended that one specifies OTP and Elixir versions
+Additionally, it is recommended that one specifies Erlang/OTP, Elixir and `rebar3` versions
 using YAML strings, as these examples do, so that numbers like `23.0` don't
 end up being parsed as `23`, which is not equivalent.
 
@@ -29,7 +34,7 @@ For pre-release Elixir versions, such as `1.11.0-rc.0`, use the full version
 specifier (`1.11.0-rc.0`). Pre-release versions are opt-in, so `1.11.x` will
 not match a pre-release.
 
-### Basic example
+### Basic example (Elixir)
 
 ```yaml
 on: push
@@ -47,7 +52,24 @@ jobs:
       - run: mix test
 ```
 
-### Matrix example
+### Basic example (`rebar3`)
+
+```yaml
+on: push
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: erlef/setup-beam@v1
+        with:
+          otp-version: '22.2'
+          rebar3-version: '3.14.2'
+      - run: rebar3 ct
+```
+
+### Matrix example (Elixir)
 
 ```yaml
 on: push
@@ -70,6 +92,28 @@ jobs:
       - run: mix test
 ```
 
+### Matrix example (`rebar3`)
+
+```yaml
+on: push
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    name: Erlang/OTP ${{matrix.otp}} / rebar3 ${{matrix.rebar3}}
+    strategy:
+      matrix:
+        otp: ['20.3', '21.3', '22.2']
+        rebar3: ['3.14.1', '3.14.3']
+    steps:
+      - uses: actions/checkout@v2
+      - uses: erlef/setup-beam@v1
+        with:
+          otp-version: ${{matrix.otp}}
+          rebar3-version: ${{matrix.rebar3}}
+      - run: rebar3 ct
+```
+
 ### Phoenix example
 
 ```yaml