Make godot-cpp installable with CMake/SCons

[ytnuf]

This PR allows this library to be installable.

So the library can be built like this: Via CMake:

cmake && make && make install

Or via SCons:

scons install

A GDExtension project can use this library like so: Via CMake:

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

Or via SCons:

env.ParseConfig("pkg-config godot-cpp --cflags --libs")

Reason

Currently the only supported way to use godot-cpp is to use as a subproject of your project. It kinda works but it's rather cumbersome because you need to rebuild godot-cpp when you build your project from scratch. This can be especially painful in CI. By allowing godot-cpp to be installable one can prevent unnecessary rebuilds. This also allows usage with a package manager (e.g vcpkg). As this also install a pkg-config file, godot-cpp can be used by projects that doesn't use neither CMake nor SCons.

• 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

[ytnuf]

With #1595 restructuring the CMake code, this PR has been changed to reflect that.

[enetheru]

@ytnuf check the recent comments here Is there anything further to add to the discussion?

@dsnopek I guess we keep these PR's around in case others want to build upon your work for their own purposes?