Feat(duckdb): add support for positional joins

[georgesittas]

Fixes #3109

I didn't add TokenType.POSITIONAL to ID_VAR_TOKENS on purpose since DuckDB treats it as a reserved keyword:

D select positional;
Error: Parser Error: syntax error at or near ";"
LINE 1: select positional;
                         ^
D select 1 positional;
Error: Parser Error: syntax error at or near "positional"
LINE 1: select 1 positional;
                 ^
D select 1 as positional;
┌────────────┐
│ positional │
│   int32    │
├────────────┤
│          1 │
└────────────┘
D with t as (select 1 as positional) select positional from t;
Error: Parser Error: syntax error at or near "from"
LINE 1: ...ect 1 as positional) select positional from t;
                                                  ^