Add the .apex extension to the Apex language

[mmanela]

Description

Adds one additional extension (.apex) for the Apex language already supported by linguist.

Checklist:

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • .apex: https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.apex+class
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • UriUtils.apex from https://code.google.com/archive/p/apex-lang/
    • Sample license(s): Apache License 2.0
  • I have included a change to the heuristics to distinguish my language from others using the same extension.
    Not Needed since there is no other langage using .apex