Export cmake targets

[zhehangd]

I use cmake to build gdextension in linux. I used to make godot-cpp a submodule in my game project with add_subdirectory to create godot-cpp targets. It works fine but is a bit cumbersome.
An alternative approach would be maintaining and building godot-cpp alone then telling my game project where the godot-cpp library is.

This PR makes cmake generate necessary files so that any other cmake-based projects can import godot-cpp like regular libraries. This makes it possible to share the compiled godot-cpp library among different projects.

Example:

# Build godot-cpp somewhere
mkdir build_godot_cpp
cmake -B build_godot_cpp -S godot-cpp -DCMAKE_BUILD_TYPE=Release
cmake --build build_godot_cpp -j12

# Build my gdextension with path to the library
mkdir build_myextension
cmake -B build_myextension -S myextension_source -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=$(realpath build_godot_cpp)
cmake --build build_myextension -j12

and in my CMakeLists.txt I write:

find_package(godot-cpp CONFIG REQUIRED)

# ...

target_link_libraries(libmyextension PUBLIC godot::godot-cpp)

[jacogasp]

This would be extremely useful in a CI/CD context, where it would be nice to avoid rebuilding godot-cpp each time.

[erodozer]

As someone trying to get godot-cpp working this way through vcpkg and would love to also see a portfile.cmake for godot-cpp in the official registry, I'm highly interested in seeing this merged.

[zhehangd]

I made some changes so we can now install the library to any directory (previously it worked directly on the build directory).

cmake -S . -B build
cmake --build build -j12
cmake --install build --prefix $(realpath install)

PS: after updating my branch I found this PR #1309. The two PRs are basically the same and the major difference is just naming.

Usage of my PR

find_package(godot-cpp CONFIG REQUIRED)
target_link_libraries(mylib PRIVATE godot-cpp::godot-cpp)

Usage of #1309

find_package(GodotCpp CONFIG REQUIRED)
target_link_libraries(MyExtension PRIVATE godot::cpp)