Version 2.7.2: Faster compiles

This patch release makes the precompiled and experimental module modes cheaper
for clients. Include-what-you-use is followed, more function bodies compile into
the library instead of into every translation unit, and the C++20 named module
now imports the precompiled implementation instead of the full header-only
library. It also fixes the Bazel precompiled build, which could give consumers
duplicate implementations.

Changed

• The C++20 named module uses the precompiled implementation and we lowered its
  floor from C++23 to C++20. CLI11::Module now works for both find_package
  and add_subdirectory, and configure fails with a clear message if CMake is
  older than 3.28. #1413
• Removed <iostream>, <codecvt>, <iomanip>, <fstream>, and <locale>
  from the declaration headers, so precompiled clients do not parse them.
  App::exit() is now three overloads instead of one with stream default
  arguments. #1411
• Moved the remaining non-template function bodies out of App.hpp,
  Option.hpp, Validators.hpp, TypeTools.hpp, ConfigFwd.hpp, and
  FormatterFwd.hpp into the impl/*_inl.hpp files, so they compile into the
  library in precompiled mode. Header-only mode is unchanged. #1415
  #1416 #1417

Fixed

• Fixed the Bazel precompiled build: CLI11_COMPILE and
  CLI11_ENABLE_EXTRA_VALIDATORS now propagate to consumers, which previously
  got duplicate implementations from both the headers and the library. Added a
  cli11_header_only target, the Windows shell32 link, and more tests.
  #1420
• Made ExtraValidators_inl.hpp use the same CLI11_ENABLE_EXTRA_VALIDATORS
  test as its header, and gave the file permission validators the
  CLI11_MODULE_INLINE marker the other global validators have. #1415

Documentation

• Documented the precompiled mode in the README. #1412
• Added a minimal module example, corrected the CLI11::Module target name in
  the book, and noted that macros such as CLI11_PARSE do not come through an
  import. #1413

Internal

• Added an iwyu preset that runs include-what-you-use over the one precompiled
  translation unit, with a mapping file for both standard libraries, and applied
  its findings. #1418