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/doc/syntax.txt

@@ -1062,6 +1062,7 @@ Variable		Highlight ~
 *c_no_cformat*		don't highlight %-formats in strings
 *c_no_c99*		don't highlight C99 standard items
 *c_no_c11*		don't highlight C11 standard items
+*c_no_c23*		don't highlight C23 standard items
 *c_no_bsd*		don't highlight BSD specific types
 *c_functions*		highlight function calls and definitions
 *c_function_pointers*	highlight function pointers definitions