When importing a mesh, allow the user to simplify the created physics TriMesh with mesh optimizer.

[basicer]

Adds a new simplification option to the model importer when the trimesh physics mesh mode is selected similar to how the options for convex decomposition are presented.

Also adds GDScript exports to generate a convex shapes and trimesh shapes (both simplified and simplified) to ImportMesh match Mesh.

Closes https://github.com/godotengine/godot-proposals/issues/3603, tested with the example project posted there.