Add .ccm, .cxxm, .c++m extensions for C++

[rdong8]

Adding the C++ module file extensions .c++m , .ccm, .cxxm to follow CMake's lead. Also made the extension list for C++ in ASCII alphabetical order (moved .cpp extension).

Description

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:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.c%2B%2Bm+module
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.ccm+module
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.cxxm+module
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • All samples are my own
    • Sample license(s):
      • MIT
  • I have included a change to the heuristics to distinguish my language from others using the same extension.
    • N/A

[lildude]

See inline comment. Additionally, I think this PR is very very premature with none of the extensions being anywhere near meeting our usage requirements (referenced in the CONTRIBUTING.md file too).

As such, I'm going to mark this as needing changes and close this PR.

Feel free to open a new PR when usage has grown to close to meeting our requirements. You'll probably want to do this in multiple PRs so one extension doesn't hold up others. For example, all the indexed .c++m files are owned by two users so that alone would block this PR assuming all the other extensions met our requirements.

Until then, users can use an override to mark the files as C++.

[lildude]

Looks like you missed the comment right at the top of this file:

linguist/lib/linguist/languages.yml

Lines 17 to 19 in d190ad8

	# extensions - An Array of associated extensions (the first one is
	# considered the primary extension, the others should be
	# listed alphabetically)

If .c++ is now considered the primary extension for the language, then this change is fine, but if it's not (which I suspect is the case) .cpp needs to remain the first extension in the list.