fix(python): throw exception with '+' on selectors

[coastalwhite]

Fixes #13986.

This PR adds an exception and hint when using the add operator between two selectors. It shows the following error:

unsupported operand type(s) for op: ('Selector' + 'Selector')

Hint: utilize the OR `|` operator for a selection union or the AND `&` operator for a selection intersection.

@stinodego can you take a look at this?

[stinodego]

I don't think this is a good idea. You can think of selectors as sets, and you cannot 'add' two sets.

We shouldn't panic of course, but rather throw a helpful error message.

I'll defer to the man himself though: @alexander-beedie

[coastalwhite]

Okay, did that now. Funnily enough, I originally wrote a patch that threw an exception but changed it for this. So it was just a matter of pressing Ctrl + Z a bunch of times.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 80.88%. Comparing base (8a6bf4b) to head (3e7307b). Report is 1423 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/selectors.py	40.00%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17117      +/-   ##
==========================================
+ Coverage   80.86%   80.88%   +0.01%     
==========================================
  Files        1456     1456              
  Lines      191141   191338     +197     
  Branches     2728     2743      +15     
==========================================
+ Hits       154562   154756     +194     
+ Misses      36073    36072       -1     
- Partials      506      510       +4     

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