fix(diff): use mmfile_t in linematch

Problem:

Linematch used to use strchr to navigate a string, however strchr does
not supoprt embedded NULs.

Solution:

Use `mmfile_t` instead of `char *` in linematch and introduce `strnchr()`.

Also remove heap allocations from `matching_char_iwhite()`

Fixes: #30505

src/nvim/linematch.h

@@ -3,6 +3,7 @@
 #include <stddef.h>  // IWYU pragma: keep
 
 #include "nvim/pos_defs.h"  // IWYU pragma: keep
+#include "xdiff/xdiff.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "linematch.h.generated.h"