Ease consumption by means of .tool-versions (#159)

* Ease consumption by means of .tool-versions

* Update as per self-review

* Update as per self-review

* Act on CI results

* Act on CI results

* Act on CI results

This value is check by code, in any case

* Act on CI results

* Act on CI results

* Act on CI results

It appears we can't install Elixir twice on Windows, but can
on Ubuntu; this is outside the scope of this branch's context
so I won't fix it now

* Tweak visual consumption elements

* Ease potential future maintenance and/or update

action.yml

@@ -11,7 +11,6 @@ inputs:
   otp-version:
     description: Version range or exact version of Erlang/OTP to use,
       or false when installing only Gleam without OTP
-    required: true
   elixir-version:
     description: Version range or exact version of Elixir to use
   gleam-version:
@@ -31,8 +30,12 @@ inputs:
       to guess versions based on semver rules
     default: loose
   disable_problem_matchers:
-    description: whether to have the problem matchers present in the results (or not)
+    description: whether to have the problem matchers present in the results
+      (or not)
     default: false
+  version-file:
+    description: a versions file (e.g. as used by `asdf`), which defines inputs
+    default: ''
 outputs:
   elixir-version:
     description: Exact version of Elixir that was installed