Unicode RL1.5 support: Implement Case-Insensitive Matching Mode

[yuuki14202028]

Summary

This PR implements case-insensitive matching support using the (?i) inline flag modifier, addressing #140.

Changes

• Added (?i) and (?-i) flag modifier parsing in Regex/Syntax/Parser/Basic.lean
• Introduced Ast.flags node to represent inline flag modifiers in the AST
• Refactored Ast.toRegexAux from index-based to state-based transformation using ToRegexState to track both group index and case-insensitivity flag
• Created Regex/Unicode/CaseFold.lean implementing Unicode Simple Case Folding (status C+S from UCD CaseFolding.txt)
• Added getCaseFoldEquivChars to generate character equivalence classes for case-insensitive matching
• Included Unicode 17.0 Simple Case Folding data in regex/data/

Implementation Notes

• Case-insensitive matching expands each character to an alternation of its case-equivalent characters (e.g., (?i)a becomes a|A)
• Uses Simple Case Folding for single-character mappings only (Full Case Folding with multi-character expansions like ß→ss is not supported)
• Flag scope follows standard regex semantics: (?i) enables and (?-i) disables case-insensitivity for subsequent patterns

Todo

• Added parser tests for (?i) and (?-i) syntax in Parser/Test.lean
• Support combining flags with non-capturing groups (e.g., (?i:abc))
• Add edge case tests for multi-character case equivalence (e.g., Σ/σ/ς, K/k/K)
• Add generate script Simple_Case_Folding.txt with .lean
• Add caseInsensitive option to Regex.parse and Regex.parse! methods
• Update correctness/ proofs to accommodate Ast.flags and ToRegexState changes
• (Optional) Support case-insensitive matching for character class ranges (e.g., (?i)[a-z])

[pandaman64]

ありがとうございます！

まだドラフトですがいくつかコメントしました。

[pandaman64]

レビュー遅れます。すいません

[pandaman64]

遅くなりましたが、マージしようと思います。コントリビューションありがとうございました！