patch 9.1.0762: 'cedit', 'termwinkey' and 'wildchar' may not be parsed correctly

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: #15811

Signed-off-by: Milly <milly.ca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    762,
 /**/
     761,
 /**/