Add support for Unity extensions

[duckdoom4]

Unity uses the extensions .jslib and .jspre to attach (pure) javascript code to WebGL applications

Description

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?q=extension%3Ajslib+NOT+nothack&type=Code
      • https://github.com/search?q=extension%3Ajspre+NOT+nothack&type=Code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://docs.unity3d.com/Manual/webgl-interactingwithbrowserscripting.html
    • Sample license(s):
  • I have included a change to the heuristics to distinguish my language from others using the same extension.