Feature tool-cache for Gleam and rebar3 (#223)

* Make iterating over mirrors more flexible (and reusable)

At the same time we fix an error, where we aren't using the
mirrors for the OTP build :)

* Make installation of cache-based tool callback -based

This hopefully eases introduction of new languages at the cost
of a bit of abstraction (there's an extensive comment on how to
fill in the install options' object)

We'll soon test this abstraction by making Gleam and rebar3
cached too

* Increase our visibility when debugging

* Improve on lessons learned to ease incorporation of further caches

(starting with Gleam on Linux)

* Cache Gleam on Windows

* Cache Rebar3 on Linux

Also, gets rid of all .sh

* Cache Rebar3 on Windows

Also, gets rid of all .ps1

* Act on CI results

And also be consistent when it comes to using
fs....Sync vs fs.promise....

* Move tests to a single file

* Move main executable to a single file

* Decrease number of changes to ease review

* Expose function required for tests

* Don't async/await when not required

* Resolve post- merge-conflict resolution issues

.eslintrc.yml

@@ -7,11 +7,12 @@ extends:
 parserOptions:
   ecmaVersion: 2022
 rules:
-  indent: [warn, 2]
+  indent: [warn, 2, {SwitchCase: 1}]
   max-len: [warn, 100]
   no-use-before-define: 0
   operator-linebreak: 0
   semi: 0
+  implicit-arrow-linebreak: 0
 
 settings:
   react: