feat: improve Unicode character handling in rectangle operations

[devin-ai-integration]

Unicode Text Handling Improvements for Rectangle Operations

This PR improves the handling of Unicode characters in rectangle operations, particularly focusing on CJK characters, emoji, and combining marks.

Changes

• Added comprehensive Unicode boundary detection for:
  • CJK characters (including full-width characters)
  • Emoji sequences
  • Combining marks and surrogate pairs
• Preserved selection direction for both horizontal and vertical selections
• Refactored character position adjustment logic for better clarity
• Added detailed test cases for various Unicode scenarios

Testing

• Added new test file src/test/suite/commands/rectangle-unicode.test.ts
• Test cases cover:
  • Japanese text boundaries
  • Greek text with combining marks
  • Emoji sequences
  • Mixed CJK (Korean/Chinese) text
• All ESLint and Prettier checks pass

Implementation Details

• Added helper functions for Unicode character detection:
  • isValidCharacterBoundary: Checks if a position is a valid Unicode character boundary
  • isSurrogatePair: Detects surrogate pairs
  • isCombiningMark: Identifies combining marks
  • isFullWidth: Detects CJK and other full-width characters
  • isEmoji: Identifies emoji characters
• Improved selection logic to respect character boundaries and maintain selection direction

Link to Devin run: https://app.devin.ai/sessions/d2255a9c45554e29943eb580739184c6

[devin-ai-integration]

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