SQLite: Add support for `==` and `is`

In SQLite it's possible to use `==` and `is` as equals operator:

```sql
select 0 == 0
-- yields 1

select 0 is 0
-- yields 1
```

Currently it yields following error:

```sql
1 == 1
   ^
(line 1, column 4):
unexpected Symbol "="
expecting scalar expression
```