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/search.c

@@ -5933,7 +5933,7 @@ static void show_pat_in_path(char_u *line, int type, bool did_show, int action,
     if (action == ACTION_SHOW_ALL) {
       snprintf((char *)IObuff, IOSIZE, "%3ld: ", count);  // Show match nr.
       msg_puts((const char *)IObuff);
-      snprintf((char *)IObuff, IOSIZE, "%4ld", *lnum);  // Show line nr.
+      snprintf((char *)IObuff, IOSIZE, "%4" PRIdLINENR, *lnum);  // Show line nr.
       // Highlight line numbers.
       msg_puts_attr((const char *)IObuff, HL_ATTR(HLF_N));
       msg_puts(" ");