Added support for adding header files to Scons (VS Projects)

Created by: marstaik

Ive added a way to add headers into a file list for Scons, which can be used to populate the Visual Studio project files correctly, to solve https://github.com/godotengine/godot/issues/34371. It can probably be used for other things as well.

It basically adds a env.add_header_files(env.****_headers, "*.h") that Scons subs can use to make sure their headers get included.

I don't know if this is the best way, but it works well. After diffing the .vcxproj file it includes 61 missing header files (excluding /drivers, as those are separate projects to be generated it seems). It took a few hours of manual work to add the command correctly everywhere it was needed.