Make godot-cpp installable with cmake config

[ytnuf]

This PR allows this library to be installable.

# after building it
cmake --install <insert_build_directory_here> --component godot-cpp-dev

The reason is so that this can be easily installed via a package manager without resorting to patches.

This project uses the standard CMAKEINSTALL* variables, so that the package manager can decide where to install the various artifacts of this package.

Also provides:

• config file so it can be found via find_package
• config-version file can the user can specify which version they require

From a user's perspective they can use this library like so:

find_package("godot-cpp" 4.2.0 CONFIG REQUIRED)
target_link_library("my-gdextension-project" PRIVATE "godot::cpp")

Note that the target library's name godot::cpp is the consistent as when used via find_package or as a sub-project.

• Bugsquad edit, closes: https://github.com/godotengine/godot-proposals/issues/9364

[AThousandShips]

Please open a proposal to track the support and details of this feature, this is pretty specific and we'd need to judge the actual demand