feat: Add is_column_selection() to expression meta, enhance expand_selector

[alexander-beedie]

Closes #16448.

Extends the existing expression meta/introspection methods with a new one that determines if a given expression exclusively selects columns. This includes selectors, bare/multi col, regex col, dtype col, use of exclude, and the positional top-level functions (first/last/nth). Aliased column selections are optionally allowed, but not by default.

This allows expand_selector to offer a new "strict=False" option such that it can additionally expand expressions like cs.starts_with("x").exclude("x!!"), which would previously raise an error.

Should cover the Great Tables use-case in the linked issue (@machow).

(Also: fixed some versionadded/changed tags as a trivial drive-by).

Example

New meta method accurately identifies all of the below:

import polars.selectors as cs
import polars as pl

for expr in (
    pl.col("colx"),
    pl.col("colx", "coly"),
    pl.col(pl.NUMERIC_DTYPES),
    cs.numeric(),
    cs.numeric().exclude("n"),
    cs.numeric() - cs.unsigned_integer(),
    pl.first(),
    pl.last(),
    pl.nth(2),
):
    assert expr.meta.is_column_selection()  # -> True

Enhanced expand_selector:

schema = {
    "id": pl.Int64,
    "desc": pl.String,
    "count": pl.UInt32,
    "value": pl.Float64,
}

Before

cs.expand_selector(schema, cs.numeric().exclude("id"))
# TypeError: expected a selector; found ...

After

cs.expand_selector(schema, cs.numeric().exclude("id"), strict=False)
# ('count', 'value')

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.43%. Comparing base (019dfe8) to head (57325f2).

Files	Patch %	Lines
crates/polars-plan/src/logical_plan/format.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16479      +/-   ##
==========================================
- Coverage   81.45%   81.43%   -0.02%     
==========================================
  Files        1409     1409              
  Lines      184961   184977      +16     
  Branches     2978     2978              
==========================================
- Hits       150656   150642      -14     
- Misses      33784    33814      +30     
  Partials      521      521              

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