Improve parser error messages with context

Description

Enhance the regex parser's error reporting to provide more helpful context and suggestions for common mistakes, making the library more beginner-friendly.

Current State

The parser has basic error types in regex/Regex/Syntax/Parser/Error.lean but error messages are quite generic:

• "unexpected character: )"
• "unexpected escaped character: q"
• "invalid range: z..a"

Proposed Implementation

Add contextual information and suggestions to error messages.

Implementation Steps

1. Enhanced Error Types:

   • Add position information to errors
   • Include surrounding context in error messages
   • Add suggestion fields for common fixes

2. Improved ToString Implementation:

   • Show position in input string where error occurred
   • Provide helpful suggestions for common mistakes
   • Format errors in a user-friendly way

3. Common Error Patterns:

   • Unmatched parentheses: suggest missing )
   • Invalid escapes: suggest valid alternatives
   • Invalid ranges: explain correct range syntax

Example Improvements

Before:

unexpected escaped character: q

After:

unexpected escaped character '\q' at position 5
  regex: "hello\qworld"
                ^
  suggestion: did you mean '\n' (newline) or remove the backslash?

Files to Modify

• regex/Regex/Syntax/Parser/Error.lean - Add new error types and better ToString
• regex/Regex/Syntax/Parser/Basic.lean - Include position info in errors

Testing

Add test cases for various error scenarios to ensure helpful messages are generated.

Skills Required

• Basic functional programming
• String formatting and manipulation
• No Lean theorem proving experience needed

Expected Outcome

Users get clear, actionable error messages when their regex syntax is invalid, improving the learning experience.