Parsing fails for LANGUAGE 'sql' in PostgreSQL CREATE FUNCTION

The following:

```sql
CREATE FUNCTION foo()
RETURNS INT
AS 'SELECT 1;'
LANGUAGE 'sql';
```

Produces parse error:

```
Syntax Error: Unexpected "'"
Was expecting to see: identifier or whitespace
--> undefined:4:10
  |
4 | LANGUAGE 'sql';
  |          ^
```

Parser expects an identifier, but string is also valid.