Add IWYU pragmas for provider header

[fnuecke]

What does this change do?

This PR suggests adding IWYU pragmas to the main provider header, toml++/toml.hpp. This allows the header to be used as intended without warnings in projects with strict clangd include diagnostics configuration.

This is relevant for cases where the single-header toml.hpp is not used, e.g. when using the library via vcpkg (where the single-header isn't even available).

I've also added stripping of these macros/comments in the script generating the single-header, since they would be incorrect in there.

Is it related to an exisiting bug report or feature request?

No.

Pre-merge checklist

• I've read CONTRIBUTING.md
• I've rebased my changes against the current HEAD of origin/master (if necessary)
• I've added new test cases to verify my change
• I've regenerated toml.hpp (how-to)
• I've updated any affected documentation
• I've rebuilt and run the tests with at least one of:
  • Clang 8 or higher
  • GCC 8 or higher
  • MSVC 19.20 (Visual Studio 2019) or higher
• I've added my name to the list of contributors in README.md

[marzer]

Great, thanks! TIL that IWYU has pragmas.