feat(python): Streamline creation of empty frame from Schema

[alexander-beedie]

Closes #20262.

Simple, but useful...

Example

import polars as pl

s = pl.Schema({"x": pl.Int32(), "y": pl.String()})
s.empty_frame()
# shape: (0, 2)
# ┌─────┬─────┐
# │ x   ┆ y   │
# │ --- ┆ --- │
# │ i32 ┆ str │
# ╞═════╪═════╡
# └─────┴─────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.65%. Comparing base (0edbfff) to head (3edcdfb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20267      +/-   ##
==========================================
- Coverage   79.65%   79.65%   -0.01%     
==========================================
  Files        1565     1565              
  Lines      218283   218286       +3     
  Branches     2475     2475              
==========================================
- Hits       173877   173873       -4     
- Misses      43839    43846       +7     
  Partials      567      567              

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[ritchie46]

We already have to_frame for Series, shall we use the same name here.

And docs entries. :)