refactor: Turn Expr.append into DSL Expr::Append

[coastalwhite]

Fixes #1535.

[ritchie46]

We should use the existing Expr::Function for this, then we don't have to implement the whole physical expressions. I don't think this operation is worth a dedicated core node.

[codecov]

Codecov Report

Attention: Patch coverage is 96.59091% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.37%. Comparing base (e52a598) to head (5f0a36a).
Report is 102 commits behind head on main.

Files with missing lines	Patch %	Lines
...ars-plan/src/plans/conversion/type_coercion/mod.rs	96.55%	2 Missing ⚠️
crates/polars-plan/src/dsl/function_expr/mod.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19536      +/-   ##
==========================================
- Coverage   79.91%   79.37%   -0.54%     
==========================================
  Files        1536     1550      +14     
  Lines      211659   214796    +3137     
  Branches     2445     2460      +15     
==========================================
+ Hits       169156   170504    +1348     
- Misses      41948    43733    +1785     
- Partials      555      559       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[coastalwhite]

We should use the existing Expr::Function for this, then we don't have to implement the whole physical expressions. I don't think this operation is worth a dedicated core node.

Done now.