add support for nested style markdowns

[FrancisMengx]

Summary
Replaces regex-based markdown parsing with a character-by-character state machine to support nested and overlapping formatting like bold italic, bold nested italic text, and strike italic through.

Key Changes
applyTextFormatting.ts: Complete rewrite using state-driven parsing instead of regex splitting
applySegmentFormatting.ts: Enhanced to handle null returns from heading adjustment
adjustHeading.ts: Now returns null for empty segments after heading removal
Tests: Added comprehensive coverage for nested formatting scenarios
Features Added
✅ Nested formatting: text, bold italic bold
✅ Overlapping styles: italic bold italic
✅ Mixed formatting: strike italic through
✅ Proper state management with toggle-based formatting
✅ Edge case handling for marker-only inputs
Technical Improvements
Character-by-character parsing for better performance
Type-safe formatting state management
Original segment preservation when no formatting applied
Null-safe segment processing
Files changed: 3 core files + 5 test files with 334 insertions, 54 deletions

[juliaroldi]

Hi @FrancisMengx, I find a small regression in this change, if you transform **test** ~~ *test* it will result in

the ~~ is causing the next word to have strikethrough without the closing characters.