Add CMake file

[enetheru]

Adding a cmake file to this template brings it inline with the updates to the godot-cpp repository.
This is ongoing work to raise the cmake solution to have feature parity.

This PR is not ready to be merged, I just wanted to get it started.

Related godot-cpp PR's

godotengine/godot-cpp#1595
godotengine/godot-cpp#1598

[enetheru]

Since this this cmake solution tracks PR#1598 I have in the godot-cpp repo, until it is merged it requires the godot-cpp module be pointed to the right revision. Locally I've been using this to make that happen:

git submodule set-url -- godot-cpp https://github.com/enetheru/godot-cpp.git
git submodule set-branch -b modernise godot-cpp
git submodule sync
git submodule update --init --recursive --remote