Problem: some API functions that check textlock (usually those that can change
curwin or curbuf) can break the cmdwin.
Solution: make FUNC_API_CHECK_TEXTLOCK call text_locked() instead, which already
checks for textlock, cmdwin and `<expr>` status.
Add FUNC_API_TEXTLOCK_ALLOW_CMDWIN to allow such functions to be usable in the
cmdwin if they can work properly there; the opt-in nature of this attribute
should hopefully help mitigate future bugs.
Also fix a regression in #22634 that made functions checking textlock usable in
`<expr>` mappings, and rename FUNC_API_CHECK_TEXTLOCK to FUNC_API_TEXTLOCK.
77118d0da8