Expand (some) ICU character classes in regex_compiler?

At the very least, getting Lower and Upper ranges would be nice, so we could
```xml
\\p{Lu}\\p{Ll}+
```
and whatnot.

If we do the "simple" thing and just expand like ranges in https://github.com/apertium/lttoolbox/blob/acx-spaces/lttoolbox/regexp_compiler.cc we get quite a lot of transitions
– https://www.compart.com/en/unicode/category/Ll (probably unreliable source) claims 2155 lowercase letters. But maybe it's OK if we keep regexes in their own `` – more research needed.

Alternatively, could/should we do something like insert a special symbol and have fst_processor treat it specially? (any tools operating on the compiled fst's like lt-trim or lt-print|hfst-txt2fst|hfst-stuff would just have to treat it opaquely)