Refactor modeling of EXCLUDE in partiql-ast and partiql-plan

[alancai98]

Description

Refactors the modeling of EXCLUDE in the partiql-ast and partiql-plan. Changes include

• EXCLUDE root modeled as a variable reference rather than an identifier symbol
• EXCLUDE wildcard steps modeled using an enum to differentiate between the two types (tuple vs collection)
• Change EXCLUDE class names in partiql-ast to align w/ the partiql-plan modeling

Other Information

• Updated Unreleased Section in CHANGELOG: [YES]

• Any backward-incompatible changes? [YES]

• Any new external dependencies? [NO]

• Do your changes comply with the Contributing Guidelines
  and Code Style Guidelines? [YES]

License Information

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

Conformance comparison report

	Base (0a75bd3)	cc70602	+/-
% Passing	92.54%	92.54%	0.00%
✅ Passing	5384	5384	0
❌ Failing	434	434	0
🔶 Ignored	0	0	0
Total Tests	5818	5818	0

Number passing in both: 5384

Number failing in both: 434

Number passing in Base (0a75bd3) but now fail: 0

Number failing in Base (0a75bd3) but now pass: 0

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (79ee005) 72.36% compared to head (434f38a) 72.36%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1311   +/-   ##
=========================================
  Coverage     72.36%   72.36%           
  Complexity     2099     2099           
=========================================
  Files           221      221           
  Lines         15974    15974           
  Branches       2892     2892           
=========================================
  Hits          11559    11559           
  Misses         3600     3600           
  Partials        815      815           

Flag	Coverage Δ
CLI	13.41% <ø> (ø)
EXAMPLES	80.28% <ø> (ø)
LANG	80.80% <ø> (ø)

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