fix(mapping): table nav mapping

[YousefHadder]

Description

Brief description of the changes.

Type of Change

• Bug fix
• New feature
• Documentation update
• Code refactoring
• Performance improvement

Related Issues

Fixes #152

Testing

• Tested manually
• Added/updated tests (if applicable)

Checklist

• Code follows project style
• Self-reviewed my code
• Commented complex logic
• Updated documentation (if needed)
• No new warnings generated

[Copilot]

Pull request overview

This PR fixes issue #152 by refactoring table navigation keymaps to use <Plug> mappings, enabling users to customize or disable the default insert mode navigation keybindings while still having access to the underlying functionality.

Key Changes:

• Added four new <Plug> mappings for insert mode table navigation (nav-left, nav-right, nav-up, nav-down) with built-in fallback to normal cursor movement
• Refactored setup_buffer_keymaps to map default keys (<A-h/j/k/l>) to the new <Plug> mappings instead of directly to navigation functions
• Added comprehensive test coverage for <Plug> mapping registration and conditional default keymap setup

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
lua/markdown-plus/table/keymaps.lua	Added <Plug> mappings for insert mode navigation with fallback behavior; refactored default keymap setup to use these <Plug> mappings
spec/markdown-plus/table_spec.lua	Added test suite for keymap registration and conditional setup based on configuration options
doc/markdown-plus.txt	Documented the four new insert mode navigation <Plug> mappings