feat(python): Allow insert_column to take expressions

[alexander-beedie]

Closes #18406.

Streamlines insertion of an Expr at an arbitrary index in a DataFrame; currently insert_column only supports this for Series, but it's straightforward to allow Expr too.

Without this affordance the user has to make two calls - one to add the new Expr (typically via with_columns), and then a second call to explicitly reorder all of the columns.

More ergonomic to allow this in a single call (this PR resolves the operation as a single select), and it makes the method consistent with with_columns, which also supports both Expr and Series input.

Example

import polars as pl

df = pl.DataFrame({
    "id": ["xx", "yy", "zz"],
    "v1": [5, 4, 6],
    "v2": [7, 3, 3],
})

df.insert_column(3, (pl.col("v1") * pl.col("v2")).alias("v3"))
df.insert_column(1, (pl.col("v2") - pl.col("v1")).alias("v0"))

# shape: (3, 5)
# ┌─────┬─────┬─────┬─────┬─────┐
# │ id  ┆ v0  ┆ v1  ┆ v2  ┆ v3  │
# │ --- ┆ --- ┆ --- ┆ --- ┆ --- │
# │ str ┆ i64 ┆ i64 ┆ i64 ┆ i64 │
# ╞═════╪═════╪═════╪═════╪═════╡
# │ xx  ┆ 2   ┆ 5   ┆ 7   ┆ 35  │
# │ yy  ┆ -1  ┆ 4   ┆ 3   ┆ 12  │
# │ zz  ┆ -3  ┆ 6   ┆ 3   ┆ 18  │
# └─────┴─────┴─────┴─────┴─────┘

[codecov]

Codecov Report

Attention: Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.

Project coverage is 79.87%. Comparing base (ab5200d) to head (885247b).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/dataframe/frame.py	72.22%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19024      +/-   ##
==========================================
+ Coverage   79.85%   79.87%   +0.02%     
==========================================
  Files        1524     1524              
  Lines      207734   207762      +28     
  Branches     2908     2913       +5     
==========================================
+ Hits       165891   165956      +65     
+ Misses      41294    41255      -39     
- Partials      549      551       +2     

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