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/mbyte.c

@@ -2613,8 +2613,8 @@ char *string_convert_ext(const vimconv_T *const vcp, char *ptr, size_t *lenp, si
 
 /// Table set by setcellwidths().
 typedef struct {
-  long first;
-  long last;
+  int64_t first;
+  int64_t last;
   char width;
 } cw_interval_T;