runtime(c): Update syntax and ftplugin files

- highlight more C keywords, including some from C23

Conditionally highlight C23 features:
- #embed, #elifdef and #elifndef preprocessor directives
- predefined macros
- UTF-8 character constants
- binary integer constants, _BitInt literals, and digit separators
- nullptr_t type and associated constant
- decimal real floating-point, bit precise and char types
- typeof operators

Matchit:
- update for new preprocessor directives

fixes: #13667
fixes: #13679
closes: #12984

Co-authored-by: Albin Ahlbäck <albin.ahlback@gmail.com>
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/c.vim

@@ -44,7 +44,7 @@ endif
 " When the matchit plugin is loaded, this makes the % command skip parens and
 " braces in comments properly.
 if !exists("b:match_words")
-  let b:match_words = '^\s*#\s*if\(\|def\|ndef\)\>:^\s*#\s*elif\>:^\s*#\s*else\>:^\s*#\s*endif\>'
+  let b:match_words = '^\s*#\s*if\%(\|def\|ndef\)\>:^\s*#\s*elif\%(\|def\|ndef\)\>:^\s*#\s*else\>:^\s*#\s*endif\>'
   let b:match_skip = 's:comment\|string\|character\|special'
   let b:undo_ftplugin ..= " | unlet! b:match_skip b:match_words"
 endif