Change modeling of Literals in the AST remove ambiguity

[jpschorr]

Change parsing and AST-modeling of literals to not share AST structures with non-scalar expressions.

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[codecov]

Codecov Report

Attention: Patch coverage is 89.07563% with 13 lines in your changes missing coverage. Please review.

Project coverage is 80.75%. Comparing base (75ffa06) to head (da6d69d).
Report is 1 commits behind head on dev-ion-doc.

Files with missing lines	Patch %	Lines
partiql-parser/src/parse/parse_util.rs	83.67%	5 Missing and 3 partials ⚠️
partiql-ast/src/ast.rs	0.00%	4 Missing ⚠️
partiql-logical-planner/src/lower.rs	94.73%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           dev-ion-doc     #517      +/-   ##
===============================================
+ Coverage        80.70%   80.75%   +0.04%     
===============================================
  Files               88       88              
  Lines            19409    19483      +74     
  Branches         19409    19483      +74     
===============================================
+ Hits             15665    15734      +69     
- Misses            3324     3326       +2     
- Partials           420      423       +3     

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[github-actions]

Conformance comparison report

	Base (75ffa06)	341e05f	+/-
% Passing	86.98%	86.98%	0.00%
✅ Passing	5564	5564	0
❌ Failing	833	833	0
🔶 Ignored	0	0	0
Total Tests	6397	6397	0

Number passing in both: 5564

Number failing in both: 833

Number passing in Base (75ffa06) but now fail: 0

Number failing in Base (75ffa06) but now pass: 0

[jpschorr]

Follow-up #518 to fix the cargo-deny failure.