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/terminal.c

@@ -1511,7 +1511,7 @@ static void adjust_scrollback(Terminal *term, buf_T *buf)
       term->sb_current--;
       xfree(term->sb_buffer[term->sb_current]);
     }
-    deleted_lines(1, (long)diff);
+    deleted_lines(1, (linenr_T)diff);
   }
 
   // Resize the scrollback storage.