Improve File Extension Logic

[graphemecluster]

• Add alternative LICENSE and README names
• Add mjs, cjs, mts, cts
• Undetect filenames without extension, e.g. py (except LICENSE and README (there should probably be more))
• Lookup level2ExtensionNames first even for filenames with more than 2 dots, e.g. foo.bar.spec.js
• Refactor to remove duplicated logic

[bummoblizard]

Thanks for cleaning up the code!