This commit changes *.h files to default to C (instead of C++), and
deprecates the existing `g:c_syntax_for_h` and `g:ch_syntax_for_h`
variables in favor of a unified `g:filetype_h`, like is used for other
ambiguous file extensions.
closes: #17135
Signed-off-by: Amelia Clarke <selene@perilune.dev>
Signed-off-by: Christian Brabandt <cb@256bit.org>