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

dist/install-elixir

@@ -2,9 +2,19 @@
 
 set -eo pipefail
 
-cd $RUNNER_TEMP
+cd "$RUNNER_TEMP"
 
-wget -q https://repo.hex.pm/builds/elixir/${1}${2}.zip
-unzip -d .setup-beam/elixir ${1}${2}.zip
-rm ${1}${2}.zip
-echo "$(pwd)/.setup-beam/elixir/bin" >> $GITHUB_PATH
+VSN=${1}
+FILE_INPUT="${VSN}.zip"
+FILE_OUTPUT=elixir.zip
+DIR_FOR_BIN=.setup-beam/elixir
+
+rm -f "${FILE_OUTPUT}"
+rm -rf "${DIR_FOR_BIN}"
+wget -q -O "${FILE_OUTPUT}" "https://repo.hex.pm/builds/elixir/${FILE_INPUT}"
+mkdir -p "${DIR_FOR_BIN}"
+unzip -q -d "${DIR_FOR_BIN}" "${FILE_OUTPUT}"
+rm -f "${FILE_OUTPUT}"
+echo "$(pwd)/${DIR_FOR_BIN}/bin" >> "$GITHUB_PATH"
+echo "Installed Elixir version follows"
+${DIR_FOR_BIN}/bin/iex -v