feat(image link): add comprehensive image link support

[YousefHadder]

Closes #117

Summary

Adds full support for Markdown image links with insert, edit, selection conversion, and toggle operations.

Features

• Insert image link: <leader>mL to insert new image with alt text, URL, and optional title
• Convert selection to image: Select text and <leader>mL to convert to image link
• Edit image link: <leader>mE to edit existing image (alt, URL, title)
• Toggle link/image: <leader>mA to convert between regular links and image links
• Smart detection: Accurately detects images with or without titles

Implementation

• Created lua/markdown-plus/images/init.lua with 387 lines
• Added 39 automated tests (all passing)
• Updated configuration, types, and validation
• Comprehensive documentation in README and vimdoc
• Manual test guide with 18 test cases

Testing

• ✅ All 39 automated tests passing
• ✅ Manual testing completed
• ✅ Code quality: 0 warnings, 0 errors
• ✅ Pattern matching for images with/without titles
• ✅ Toggle preserves title attributes

Documentation

• Updated README with feature list, examples, configuration, and keymaps
• Updated vimdoc with complete usage guide
• Created MANUAL_TEST_IMAGES.md for manual testing

[Copilot]

Pull Request Overview

This PR adds comprehensive image link support to markdown-plus.nvim, providing insert, edit, selection conversion, and toggle operations for Markdown image syntax (![alt](url "title")).

• Implements full image link management with smart detection of images with/without title attributes
• Adds 4 new keymaps for image operations in normal and visual modes
• Updates configuration, types, validation, and comprehensive documentation

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
spec/markdown-plus/images_spec.lua	New test file with 39 tests covering pattern matching, cursor detection, toggling, and edge cases; includes placeholder tests for user input functions
lua/markdown-plus/types.lua	Adds images boolean field to feature configuration types
lua/markdown-plus/init.lua	Integrates images module into plugin initialization and autocmd setup
lua/markdown-plus/images/init.lua	New 386-line module implementing all image link operations with patterns, keymaps, and user input functions
lua/markdown-plus/config/validate.lua	Adds validation for images feature flag
doc/markdown-plus.txt	Adds comprehensive vimdoc section with usage examples and keymap documentation
README.md	Adds feature overview, detailed examples, configuration options, and keymap reference