Utilities/Sphinx: Improve Pygments CMake lexer

• Add a "dummy" pyproject.toml to simplify installing needed tools w/ poetry
• Cleanup cmake.py code according to basic linters
• Extract CMakeLexer to a separate file for easy testing
• Introduce tests for the CMake lexer
• CMake lexer has been rewritten from scratch (now it's stateful) to fix the following cases:
  • <blah | blah> placeholders (e.g., see string(JSON…)
  • nested parenthesis now highlighted properly (e.g., if((A OR B) AND (C OR D)))
  • named arguments containing placeholders (e.g., <LANG>_STANDARD in try_compile())
  • bracket comments (e.g., see cmake-language.7)
  • bracket strings (e.g., see cmake-language.7)
• Also the lexer learned new tokens and some inappropriate ones have been removed
  • genexes now highlighted
  • string escape characters now highlighted
  • strings interpolation support has improved
  • ENV/CACHE/DATA highlighting has improved
• Some pages have been fixed to enable CMake code blocks instead of unparsed literals (cuz it was a "syntax" error for the previous lexer)
• Since the new lexer supports bracket strings some signatures have been fixed to disambiguate [[ usage

Topic-rename: sphinx-pygments-lexer