refactor: use uint8_t for blobs and ga_append() (#21916)

A blob is used as a sequence of bytes and usually accessed individually,
not as as a NUL-terminuated string, so uint8_t should be better.

Not sure about ga_append(), but using uint8_t leads to fewer casts.

src/nvim/mapping.c

@@ -1199,7 +1199,7 @@ static char_u *translate_mapping(char_u *str, int cpo_flags)
     }
 
     if (c) {
-      ga_append(&ga, (char)c);
+      ga_append(&ga, (uint8_t)c);
     }
   }
   ga_append(&ga, NUL);