feat: Add cat.slice

[mcrumiller]

As with the other categorical string operations this doesn't allow expression input, as that would negate the whole benefit from performing the operation on categoricals.

>>> import polars as pl
>>> s = pl.Series(["foobar", "barfoo", "x", None], dtype=pl.Categorical)
>>> s.cat.slice(1, 3)
shape: (4,)
Series: '' [str]
[
        "oob"
        "arf"
        ""
        null
]

[codecov]

Codecov Report

Attention: Patch coverage is 97.72727% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.47%. Comparing base (10dd3dc) to head (b29fccd).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/dsl/function_expr/cat.rs	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21971      +/-   ##
==========================================
- Coverage   80.47%   80.47%   -0.01%     
==========================================
  Files        1636     1636              
  Lines      236598   236626      +28     
  Branches     2693     2693              
==========================================
+ Hits       190413   190428      +15     
- Misses      45551    45564      +13     
  Partials      634      634              

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[orlp]

Thanks :)