refactor(python): make structify behaviour experimental, while also extending it to aliased expressions

[alexander-beedie]

Closes #6594.

Pending additional consideration (and moving the logic into Rust), auto-structification of multi-output expressions has been...

• moved behind an experimental Config setting.
• extended to regular aliased expressions, in addition to kwargs.

Example:

import polars as pl
pl.Config.set_auto_structify(True)

df = pl.DataFrame({
    "x1": [1,4,5],
    "x2": [1,2,3],
}).with_columns(
    pl.col(["x1","x2"]).pct_change().alias("pct_change"),
)

# shape: (3, 3)
# ┌─────┬─────┬─────────────┐
# │ x1  ┆ x2  ┆ pct_change  │
# │ --- ┆ --- ┆ ---         │
# │ i64 ┆ i64 ┆ struct[2]   │
# ╞═════╪═════╪═════════════╡
# │ 1   ┆ 1   ┆ {null,null} │
# │ 4   ┆ 2   ┆ {3.0,1.0}   │
# │ 5   ┆ 3   ┆ {0.25,0.5}  │
# └─────┴─────┴─────────────┘

Update:

Also fixes inconsistent str-to-col behaviour between kwarg / non-kwarg exprs, caused by a missing str_to_lit=False param.