Merge 'Add support for nullif scalar function' from Kim Seon Woo

### EXPLAIN nullif(1,2)
<img width="1339" alt="image" src="https://github.com/user-attachments/assets/08230797-914d-4922-b52c-5b2b2b4c2a12">

### Issue
https://github.com/penberg/limbo/issues/144

Closes #299

testing/scalar-functions.test

@@ -279,6 +279,18 @@ do_execsql_test max-null {
   select max(null,null)
 } {}
 
+do_execsql_test nullif {
+  select nullif(1, 2)
+} {1}
+
+do_execsql_test nullif {
+  select nullif(1, 1)
+} {}
+
+do_execsql_test nullif {
+  select nullif('limbo', 'limbo')
+} {}
+
 do_execsql_test substr-3-args {
   SELECT substr('limbo', 1, 3);
 } {lim}