Feature: Add Undo/Revert Functionality with Ctrl+Z

[susbakun]

Summary

This PR implements undo/revert functionality for the editor, allowing users to undo their last edit operation using Ctrl+Z. The feature supports reverting all edit operations including character insertions, deletions (both forward and backward), newlines, and range deletions.

Features

• ✅ Undo with Ctrl+Z: Press Ctrl+Z to revert the last edit operation
• ✅ Revert buffer: Maintains a history of up to 100 edit operations
• ✅ Full operation support:
  • Character insertions
  • Character deletions (Delete key)
  • Backward deletions (Backspace)
  • Newline insertions
  • Range deletions (selected text)
• ✅ Multi-character grapheme support: Correctly handles Unicode graphemes that span multiple characters
• ✅ Proper text extraction: Range deletions preserve indentation and formatting when reverted

Implementation Details

Key Changes

1. New Edit::Revert command: Added to the Edit enum, triggered by Ctrl+Z
2. New Edit::InsertText variant: Added to support reverting multi-character deletions and range deletions
3. Revert buffer: Added revert_buffer: Vec<RevertCommand> to View struct to store edit history
4. Text extraction methods:
   • Buffer::get_a_grapheme(): Extracts a single grapheme at a location
   • Buffer::get_range_grapheme(): Extracts text from a selection range
   • Line::get_a_grapheme(): Public method to extract graphemes from a line
   • Line::get_grapheme_range(): Public method to extract grapheme ranges from a line
5. Skip revert buffer flag: Added skip_revert_buffer parameter to prevent infinite loops when applying revert commands

Technical Highlights

• Grapheme-aware: Uses grapheme indices instead of byte indices to correctly handle Unicode characters
• Memory management: Limits revert buffer to 100 entries to prevent unbounded memory growth
• Cursor position restoration: Restores cursor to the correct location when reverting operations

Usage

1. Perform any edit operation (insert, delete, etc.)
2. Press Ctrl+Z to undo the last operation
3. Repeat Ctrl+Z to undo previous operations (up to 100)