Unicode RL1.1 support: Add \u{...} Hex Notation for Full Unicode Range

[yuuki14202028]

This PR implements the \u{...} escape sequence to support the full Unicode range, addressing #136.

Changes

• Updated Regex.Syntax.Parser to handle variable-length hex sequences enclosed in braces (e.g., \u{1F600}).
• Added validation to ensure code points are within the valid Unicode range (0x0 to 0x10FFFF) and are not surrogate pairs.
• Preserved existing support for fixed-width \uXXXX and \xXX escapes.
• Added comprehensive test cases in regex/Regex/Syntax/Parser/Test.lean from Issue.

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Note

Unicode escape support and validation

• Add \u{...} parsing in escapedChar via hexEscape/unicodeEscape supporting 1–6 hex digits, falling back to \uHHHH; preserves \xHH and \uHHHH
• Validate code points with isValidChar; introduce errors invalidCodePoint and tooManyHexDigits with messages in Error.lean
• Extend tests to cover valid ranges (ASCII, BMP, supplementary planes), lowercase digits, variable lengths, and error cases (empty braces, too many digits, invalid hex, missing } , out-of-range and surrogate code points)

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[pandaman64]

PRありがとうございます！全体的には良いのですが、細部についてコメントしたので取り入れてもらえると助かります。

[pandaman64]

LGTMです。コントリビューションありがとうございます！