Fix parser to support advanced Modelica features from Chua Circuit example

[ChrisRackauckas-Claude]

Summary

This PR addresses parsing issues identified in GitHub issue #8, specifically with the Chua Circuit example that includes advanced Modelica language features that weren't previously supported by the parser.

Changes Made

1. Comment Prefix Support

• Issue: Parser failed on files with comment headers like //! base 0.1.0
• Fix: Modified base_modelica rule to accept Star(LINE_COMMENT) before package declarations
• Impact: Now supports OpenModelica-style version comments

2. Final Keyword Support

• Issue: Parser didn't recognize the final parameter modifier
• Fix:
  • Extended type_prefix grammar to include optional final keyword
  • Updated BaseModelicaTypePrefix AST node to include final_flag field
  • Updated constructor to handle final keyword parsing
• Impact: Supports final parameters like final parameter Boolean flag = false

3. Complex Parameter Modifications

• Issue: Parser failed on parameters with multiple attributes
• Fix: Fixed argument_list rule to include proper whitespace handling between comma-separated arguments
• Impact: Now supports complex parameter declarations like:

  parameter Real 'T_ref'(nominal = 300.0, start = 288.15, min = 0.0, unit = "K") = 300.15

4. Annotation Parsing Improvements

• Issue: Annotations with content caused constructor argument mismatches
• Fix: Improved comment.matcher to properly combine string comments and annotations
• Impact: Supports annotations like annotation(Evaluate = true)

5. If-Expression Parsing

• Issue: Nested if-then-else expressions caused infinite parser loops
• Fix: Improved if_expression to support both elseif and else if syntax with safeguards
• Impact: Better support for conditional expressions (though else if still has some limitations)

Testing

Added comprehensive test cases that verify each feature works individually:

• ✅ Comment prefix before package
• ✅ Final parameter keyword
• ✅ Complex parameter modifications
• ✅ Annotation parsing
• ✅ Assert functions (already worked)
• ✅ Conditional expressions with elseif

Impact

These changes significantly expand the parser's capabilities, enabling it to handle more realistic Base Modelica code from the Modelica Standard Library. The Chua Circuit example can now be parsed successfully (with minor syntax adjustments for else if -> elseif).

Test Plan

• Verify all individual features parse correctly
• Test with simplified Chua Circuit example
• Ensure existing Newton Cooling example still works
• Full integration testing with complete Chua Circuit (pending else if fix)

Addresses #8

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