assert() is compiled out for release builds, but we don't want to
continue running in these impossible situations.

This also resolves the "implicit fallthrough" warnings for the asserts
in switch cases.

"has_mbyte" always evaluates to "true".
Continue dead code removal, started in https://github.com/neovim/neovim/pull/13275.

* Fix multiple typos in `src/nvim/api/ui_events.in.h`
* Below spellings are corrected:
  * Definitions
  * Revision
  * Performance

* Typo fix `presense` should be presence

* Typo fix, follwed -> followed

* Typo fix, attemping -> attempting

* Typo fix, duplicate 'the'

* Typo fix, bounaries -> boundaries

* Revert "Typo fix, bounaries -> boundaries"

This reverts commit 24779e55e596f7534585938a7e532830ad2fa83c.

* Revert "Typo fix, duplicate 'the'"

This reverts commit 1d41d74f78736e67821a15edc9c174a2c3024f87.

* Revert "Typo fix, attemping -> attempting"

This reverts commit d8b2860dfcac366af7afae1f09946ed0aee4a34e.

- K_KORIGIN instead of K_KCENTER: This name is similar to what is used
  by xev. Alternative could be K_KBEGIN as hinted here:
  https://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h2-PC-Style-Function-Keys
  But I find Begin and Home too similar, and it might induced some
  confusion. The naming looked related to some old keyboard
  configuration.

- keymap.c: alias KPPeriod to kDel instead of kPoint.
  This might seems weird, but this is actually the behaviour that should
  be expected. libtermkey produces "KPPeriod" when num lock is off. To
  fix this would need to change this name in termkey.

closes #9780
closes #9793

Note: there are three changes to ascii_isident. Reverting first two (in 
find_special_key and first in get_special_key_code) normally fails the new test 
with empty &isident, but reverting the third does not. Hence adding `>` to 
&isident.

Ref vim/vim#2389.