refactor: Refactor (group-)rule to be lazily evaluated

[delsner]

Motivation

Addresses #59.

Changes

We don't want to introduce a separate decorator as done in #60 with dy.lazy_rule.
Instead this PR instead lazily evaluates (group-)rules and gets rid of the column check which requires all symbols used inside the rule expression to be defined (including the schema class itself).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (c25880e) to head (93364f3).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #64   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           41        41           
  Lines         2153      2143   -10     
=========================================
- Hits          2153      2143   -10     

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[borchero]

I think we still have to remove/simplify the RuleImplementationError for coverage. Feel free to get rid of the entire comment; the implementation in the polars plugin is trivial (I already tried), no need for keeping this stuff around.

[borchero]

Thanks! 🚀