Add '.cppm' files as C++ sources.

[robert-andrzejuk]

It is becoming common to use "cppm" or "ixx" as extension for C++ modules. This is standard C++ code.
"ixx" is already on the list.

Reference: https://gitlab.kitware.com/cmake/cmake/-/merge_requests/5926

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=path%3A*.cppm&type=code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/kaladron/galactic-bloodshed/blob/651676152d2f21b380ef1c3e109edc0d33e622df/gb/gblib.cppm#L4
    • Sample license(s):
      • https://github.com/kaladron/galactic-bloodshed/blob/master/LICENSE
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

Please read and follow the instructions in the CONTRIBUTING.md file for adding a new extension. Please also fill the approproate section of the template in the OP.

[robert-andrzejuk]

@lildude
I edited my post to add the link to the github query: https://github.com/search?q=path%3A*.cppm&type=code
Is that all that is required?

[lildude]

Nope. You have missed the point in the CONTRIBUTING.md file about preferring real world samples. You've clearly added a "hello world" sample which is never considered real world code. Please add a real world sample and update the OP to link to the source and state the license of the sample.

[robert-andrzejuk]

Ok. Thanks for you help.

I copied a file from a repository. And attached it in samples/C++ section.
Also updated my original post (OP). (OP was an unknown abbreviation to me, till now.)