build: enable MSVC level 3 warnings (#21934)

MSVC has 4 different warning levels: 1 (severe), 2 (significant), 3
(production quality) and 4 (informational). Enabling level 3 warnings
mostly revealed conversion problems, similar to GCC/clang -Wconversion
flag.

src/nvim/search.c

@@ -1031,7 +1031,7 @@ int do_search(oparg_T *oap, int dirc, int search_delim, char *pat, long count, i
   struct soffset old_off;
   int retval;                   // Return value
   char *p;
-  long c;
+  int64_t c;
   char *dircp;
   char *strcopy = NULL;
   char *ps;