Add CMake custom commands for generated contents

[gemmaro]

Hello, this addresses #580 by using CMake features to generate some support files.

The existing workflow doesn't change.
Makefile tasks for src/{scanners.c,case_fold.inc,entities.inc} are kept as is, and the added CMake custom commands are only invoked when python3, re2c, and clang-format are installed.

I added the .clang-format file to let clang-format share the consistent behavior across Makefile and tool scripts.

I referred the following resources:

• CMake documentation: add_custom_command (and its example usage), if conditionals, find_program, (add_custom_target though I didn't use this time)
• Similar file generation with CMake in other projects: cmigemo
• Clang-Format Style Options

Thank you,

[nwellnhof]

AFAICT, this will rebuild the generated code on each build (if the tools could be found.) This is not what we want. Also see my comments in #580.

[AndersonTorres]

@gemmaro have you tried to use add_custom_target instead?
For us at Nixpkgs and Guix, it would be sufficient. IIRC they generate targets in the backend that can be called independently later via Make or Ninja.

[gemmaro]

@AndersonTorres Yes I have. I think it is a good idea if it is more or less valuable for cmark maintainers as well.