Convert template_preprocess in form.inc

I was tempted to also move form_select_options() as it's a function that in core is only used from template_preprocess_select(), but there are direct calls in contrib, so we might want to move it to FormHelper or so and that is then definitely out of scope.

Similar with form_get_options(), which is actually not called at all in core or contrib and I think we should deprecate that without replacement.

Beside those two, the only thing left in form.inc is then batch functions.

I have done a partial review:

I've confirmed all deprecations are 11.3 -> 12

All are __function__

All functions call the right method
All initial preprocess call the right method
All comments point to the new method

Need to run the git command to confirm the body of the methods align.

Ok I had a chance to finish my review with: git diff 11.x --color-moved=dimmed_zebra --color-moved-ws=ignore-all-space
See #3518822-14: Convert template preprocess hooks in core/includes/theme.inc → for an explanation.

With that and my review above this is RTBC.

The only line length changes for Codesniff are the comment in the media module.