Fixes to the Test Suite, and a Dependency Refactor

[kkohrt]

A recent cleanup PR removed a redundant accessor from a file (module.rb) that actually used it, which made it hard to follow the code path.
It was ultimately a final step in a chain of cleanup actions in execjs.rb.
This PR basically unwinds a much earlier refactor of execjs.rb here that was written so that:

You can now load ExecJS and bypass engine autodetection by requiring "execjs/runtimes" instead of "execjs".

But that same problem was re-addressed here in order to:

prevent runtime autodetect on require

which now left module methods defined in two places :-(

This PR

• Glues it all back together again into an single module definition

• Fixes some predicate methods to return false, not nil

• Fixes a warning in the test suite

• Uses File.join rather than concatenating strings for path definitions (since Windows is supported?)

• Does some quick fixes on spec tests and CI config to get everything to pass ✅
  These could probably use some additional refinement, but I don't have the setup to do it locally hey! I think I got github actions running in my fork. I will try to tighten up the code wherever I can in the coming days; and happy to take any suggestions in the meantime.

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NOTE:
A future TODO item would be to clean up this section of ExternalRuntime so that either (a) the switchard is inside a lightweight external_exec method than then calls either static or dynamically created platform-specific methods to actually do the work; or (b) external_exec is defined only when needed at runtime using method_missing, and again, executes the switchyard logic at that time rather than when the class is loaded.