feat: Add len method to arr

[coastalwhite]

I did a bit of an oopsie on the original PR #21513. I cannot seem to get that one back. Here it is again.

[codecov]

Codecov Report

Attention: Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.24%. Comparing base (e801b83) to head (94cb400).
Report is 57 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/dsl/function_expr/array.rs	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21618      +/-   ##
==========================================
+ Coverage   79.81%   80.24%   +0.43%     
==========================================
  Files        1592     1604      +12     
  Lines      229266   231361    +2095     
  Branches     2623     2639      +16     
==========================================
+ Hits       182991   185658    +2667     
+ Misses      45670    45094     -576     
- Partials      605      609       +4     

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