comment: C, Cpp and Css styles reorganization (big)

[Thannoy]

C and C++ features different comment styles:

• only multi-line /* */ comment is valid in traditional C89
• C++ also supports // single line comments from its beginning
• C started to support C++-style // comments in C99

For this reason, multi-line /* */ is often named C-style comment and single-line // is often named C++-style comment.

In current implementation, there were only a "CCommentStyle" with /* */ and // support, used for both C and C++ files (also used by quite many other file types).

Therefore:

• Rename CCommentStyle as CppCommentStyle
• Create a new CCommentStyle with only /* */ support
• Remove redundant CssCommentStyle (same as new CCommentStyle)
• Associate *.c and *.h with CCommentStyle

Caution:

• this (slightly) changes *.c and *.h comments (by removing their multi-line support)
  • despite *.h files being often used also for C++, which is a minor issue in my opinion since C comments is a subset of C++ comment
• this changes the list of known style shorthands:
  • "c" style shorthand meaning changes
  • "css" style shorthand is no more known (want we keep a redundant CssCommentStyle to avoid this?)

This PR is a little bit big but in my opinion results in a clearer handling of C/C++ (and Css).

[Thannoy]

Another caution about this PR: since it touches quite many places, conflicts with other PRs are likely.

[Thannoy]

• Resync'ed with main 1a131cc
• added missing cppsingle shorthand update

[Thannoy]

For information purpose, resyncing this PR with master updates is not that trivial. Here is my workflow:

• git fetch
• git merge origin/main
• re-do comment.py d073a5 key modifications:
  • git checkout origin/main -- src/reuse/comment.py
  • rename all occurrences of CCommentStyle to CppCommentStyle
  • rename all occurrences of CssCommentStyle to CCommentStyle
  • fix impacted *CommentStyle themselves and their shorthands (C/Cpp/CppSingle/Css)
  • change .c and .h comment style to CCommentStyle
• tests

[carmenbianca]

Hi @Thannoy and thanks! I rebased on top of master again, and did some more adjusting. To maintain support for the old shorthands, I manually added them to the dictionary and added tests for this.

This PR is technically a breaking change still, but I can live with that.

Thanks for the work and the history context.