Add context menu item to prettify a table at the cursor

[darkriszty]

Add a new context menu entry for formatting individual Markdown tables without requiring a text selection or without formatting the entire document. When right-clicking inside a table, the context menu should contain "Prettify markdown table at cursor" to format just that table.

Key considerations:

• Smart Visibility: The context menu item should only appear when the cursor is positioned within a valid table.
• Performance: Efficient table detection that searches locally around the cursor + some caching to avoid recomputations when nothing changes.