vim-patch:9.1.0762: 'cedit', 'termwinkey' and 'wildchar' may not be parsed correctly (#30704)

Problem:  'cedit', 'termwinkey' and 'wildchar' may not be parsed
          correctly
Solution: improve string_to_key() function in option.c
          (Milly)

- Problem: `^@` raises an error.
  Solution: Store as `<Nul>`.
- Problem: `<t_xx` does not raise an error.
  Solution: Raise an error if closing `>` is missing.
- Problem: Single `<` or `^` raises an error. It is inconvenient for users.
  Solution: They are stored as a single character.

closes: vim/vim#15811

a9c6f90918

Co-authored-by: Milly <milly.ca@gmail.com>

src/nvim/options.lua

@@ -9620,7 +9620,12 @@ return {
         Some keys will not work, such as CTRL-C, <CR> and Enter.
         <Esc> can be used, but hitting it twice in a row will still exit
         command-line as a failsafe measure.
-        Although 'wc' is a number option, you can set it to a special key: >vim
+        Although 'wc' is a number option, it can be specified as a number, a
+        single character, a |key-notation| (e.g. <Up>, <C-F>) or a letter
+        preceded with a caret (e.g. `^F` is CTRL-F): >vim
+        	:set wc=27
+        	:set wc=X
+        	:set wc=^I
         	set wc=<Tab>
         <
       ]=],