feat(core): fix UMR in Uset 🙀

- USet struct was depending on the COMP_KEYBOARD_EX (the .kmx data), but was used after load completed during processing
- Change the USet structure to contain a std::list<COMP_KMXPLUS_USET_RANGE> i.e. copying it
- but, while we're at it, added functions to validate that characters are within valid Unicode ranges.

#7375