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

action.yml

@@ -1,6 +1,9 @@
-name: Setup Elixir
-description: Set up a specific version of OTP and Elixir and add the command-line tools to the PATH
-author: GitHub
+---
+name: Setup Erlang/OTP with optional Elixir and/or rebar3
+description: >
+  Set up a specific version of Erlang/OTP, Elixir, and/or rebar3
+  and add the command-line tools to the PATH
+author: Erlang Ecosystem Foundation
 branding:
   color: blue
   icon: code
@@ -8,18 +11,22 @@ inputs:
   elixir-version:
     description: Version range or exact version of Elixir to use
   otp-version:
-    description: Version range or exact version of OTP to use
+    description: Version range or exact version of Erlang/OTP to use
   install-hex:
     description: Whether to install Hex
     default: true
   install-rebar:
     description: Whether to install Rebar
     default: true
+  rebar3-version:
+    description: Version range or exact version of rebar3 to use
 outputs:
   elixir-version:
     description: Exact version of Elixir that was installed
   otp-version:
-    description: Exact version of OTP that was installed
+    description: Exact version of Erlang/OTP that was installed
+  rebar3-version:
+    description: Exact version of rebar3 that was installed
 runs:
   using: node12
   main: dist/index.js