feat: Expose returns_scalar to map_elements

[ritchie46]

fixes #17505

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.65%. Comparing base (f304a0c) to head (d6d689a).

Files	Patch %	Lines
py-polars/polars/datatypes/_parse.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17613   +/-   ##
=======================================
  Coverage   80.64%   80.65%           
=======================================
  Files        1484     1484           
  Lines      195509   195511    +2     
  Branches     2780     2781    +1     
=======================================
+ Hits       157675   157692   +17     
+ Misses      37324    37308   -16     
- Partials      510      511    +1     

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