[3.x] Mesh merging - refactor to be backward compatible for CPU / GPU storage

[lawnjelly]

Allows the old merge_meshes() function to work from the editor.

Fixes #91988

Most of the lines changed here are just moving the implementation functions to a new location. The bug fix is allowing them to be called choosing storage CPU or GPU etc, and defaulting the old function to store in GPU (which is the old way in 3.5).

Notes

• MergeGroups totally changed the backend for merging, which inadvertently broke the old merge_meshes() function in MeshInstance when used from the editor.
• The breakage was simply because the new method defaulted to creating the result mesh on the CPU only (as this is significantly faster, as it removes a round trip to the GPU + drivers for any geometry processing).
• This PR moves the wrappers for the MeshInstance merging functions to the MergingTool, and converts the results from the wrappers to GPU meshes, so they are suitable for use in the editor and rendering.