Second stabilisation pass of `*::char`

cc #19260 

The casing transformations are left unstable (it is highly likely to be better to adopt the proper non-1-to-1 case mappings, per #20333) as are `is_xid_*`.

I've got a little todo list in the last commit of things I thought about/was told about that I haven't yet handled (I'd also like some feedback).