fix(multibyte): handle utf-8 multibyte characters in text manipulation operations

[YousefHadder]

Fixes #144 footnote, link, image insertion, and list operations corrupting multibyte UTF-8 characters (Chinese, emoji, etc.) by using character-level string splitting instead of byte-level operations.

[Copilot]

Pull request overview

This PR fixes a critical bug where text manipulation operations (list handling, link/image/footnote insertion) were corrupting UTF-8 multibyte characters (Chinese, emoji, etc.) by incorrectly using byte-level string operations instead of character-aware operations.

Key Changes

• Introduced two new UTF-8-safe utility functions (split_at_cursor and split_after_cursor) that properly handle character boundaries using vim.fn.charidx and vim.fn.byteidx
• Updated all text manipulation operations across the codebase to use these new utilities instead of direct byte-level string.sub() calls
• Added comprehensive test coverage (142 new test cases) for the utility functions covering ASCII, Chinese characters, emoji, and edge cases

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
spec/markdown-plus/utils_spec.lua	Adds 142 lines of comprehensive UTF-8 tests for both new utility functions, covering ASCII, Chinese, emoji, and edge cases
lua/markdown-plus/utils.lua	Implements two new UTF-8-safe string splitting functions with proper character boundary detection using vim functions
lua/markdown-plus/list/handlers.lua	Updates handle_enter, continue_list_content, and handle_tab to use UTF-8-safe splitting for list content manipulation
lua/markdown-plus/links/init.lua	Updates insert_link to use UTF-8-safe splitting and corrects cursor positioning after insertion
lua/markdown-plus/images/init.lua	Updates insert_image to use UTF-8-safe splitting and corrects cursor positioning after insertion
lua/markdown-plus/format/init.lua	Refactors get_word_boundaries to iterate by characters instead of bytes, with proper UTF-8 boundary detection
lua/markdown-plus/footnotes/insertion.lua	Updates insert_footnote to use UTF-8-safe splitting for reference insertion