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/extmark.c

@@ -499,8 +499,8 @@ void extmark_apply_undo(ExtmarkUndoObject undo_info, bool undo)
 }
 
 /// Adjust extmark row for inserted/deleted rows (columns stay fixed).
-void extmark_adjust(buf_T *buf, linenr_T line1, linenr_T line2, long amount, long amount_after,
-                    ExtmarkOp undo)
+void extmark_adjust(buf_T *buf, linenr_T line1, linenr_T line2, linenr_T amount,
+                    linenr_T amount_after, ExtmarkOp undo)
 {
   if (curbuf_splice_pending) {
     return;