fix(expr-ir): Allow struct.field to select from it's own context

[dangotbanned]

Description

The equivalent of this was previously raising:

>>> import polars as pl
>>> pl.select(pl.lit({"a": 1, "b": 2}).struct.field("b"), eager=False).collect()
shape: (1, 1)
┌─────┐
│ b   │
│ --- │
│ i64 │
╞═════╡
│ 2   │
└─────┘

Related issues

• Follow-up to a bug found in feat(expr-ir): Add top-level select #3647
• Child of feat(RFC): A richer Expr IR #2572