refactor: change type of linenr_T from long to int32_t

The size of long varies depending on architecture, in contrast to the
MAXLNUM constant which sets the maximum allowable number of lines to
2^32-1. This discrepancy may lead to hard to detect bugs, for example
https://github.com/neovim/neovim/issues/18454. Setting linenr_T to a
fix maximum size of 2^32-1 will prevent this type of errors in the
future.

Also change the variables `amount` and `amount_after` to be linenr_T
since they're referring to "the line number difference" between two
texts.

src/nvim/lua/stdlib.c

@@ -89,7 +89,7 @@ static int regex_match_line(lua_State *lstate)
   }
 
   long bufnr = luaL_checkinteger(lstate, 2);
-  long rownr = luaL_checkinteger(lstate, 3);
+  linenr_T rownr = (linenr_T)luaL_checkinteger(lstate, 3);
   long start = 0, end = -1;
   if (narg >= 4) {
     start = luaL_checkinteger(lstate, 4);