[WIP] Physics interpolation (2D): Add support for CPUParticles2D

[rburing]

This is the beginning of a port of @lawnjelly's

• [3.x] Physics Interpolation: Add support for CPUParticles2D #80176

to the master branch.

TODO:

• Actually port the MultiMesh interpolation.
• Port any follow-up fixes.

[lawnjelly]

It's a while since I did this but look carefully at:
#80931

I decided in retrospect that the better solution for particles (in most cases) was to process them in _process() rather than _physics_process(), and add the ability to follow an interpolated target. This is now the default in 3.x.

If you go for this approach only, it means you wouldn't need to add so much modification (as doing proper physics interpolation is more involved). So it might be worth considering trying out this simpler approach first as a test, and see how users get on with it.