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/syntax/testdir/dumps/c_preprocessor_conditional_00.dump

@@ -0,0 +1,20 @@
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