Use relative paths for the .gdextension file such that the extension can be placed in any folder.

[Ivorforce]

If we use absolute paths (as is done now), the GDExtension will only correctly load if placed in res://bin, but if we use relative paths, it will correctly load from any location.

I think using relative paths is more appropriate as it's less error prone if any user wants to modify the path from bin/ to somewhere else. Commonly, for example, GDExtensions will be placed in addons/extensionname.
This is technically not important for the demo project, as we don't expect the extension to be moved around there, but the modified .gdextension is the only one we currently have, and there's no real downside to the change.

I opened this PR this after writing an answer to this proposal. I think the described issue may be common enough to warrant giving an example of how best to do it in the template.

I'm using ./ to start relative paths to avoid this unresolved issue: godotengine/godot-cpp#1681

[dsnopek]

Discussed at the GDExtension meeting, and we think this makes sense because different people have different opinions about where GDExtensions should live in a project, and this allows them to freely move it.

[paddy-exe]

Thanks!