Separate setup-godot-cpp action from build action, to allow for more customizability

[Ivorforce]

This is coming from the same idea as #66, but proposes a different approach to achieve customizability.

I think I like this one more, since a lot of parameters were only there to be passed to scons unchanged. There's a reason github actions often follow the setup-xxx formula - it's just a good fit, I think.

I don't think the build action is needed still in this PR, but I kept it around for backwards compatibility.

[dsnopek]

Moving this to its own action is fine, and I think the name is OK.

However, while this is still part of the godot-cpp-template repo, I don't think we should be looking at this in terms of "customizability" and "backwards compatibility". This is a template that folks are meant to copy once, and then do whatever they want with. The way to customize it is to change the action itself, and there is no backwards compatibility because no one should be pulling in future changes from the template.

If we want to have a reusable action, then I think this should be moved to its own repo and maintained outside of the template project.

[Ivorforce]

As per GDExtension meeting, we will move the github setup action to godot-cpp. This will be better represented by another PR.