[FEATURE] Vim mode: change cursor appearance based on insert/normal mode

Problem Statement

When using vim editor mode, the cursor appearance doesn't change between insert and normal modes. In true vim editors, the cursor typically changes from a block (normal) to a beam/bar (insert), providing visual feedback about the current mode.

Proposed Solution

Add cursor appearance configuration that adapts based on vim mode:

• Normal mode: block cursor
• Insert mode: beam/bar cursor

This could be controlled via settings.json and the /settings UI, or automatically when editorMode: vim is configured.

Related Issues

• #20878 - General cursor style customization request
• #16086 - Earlier cursor style configuration request
• #10215 - Earlier cursor style customization request

Use Case

Vim users expect visual mode indicators. This helps users quickly identify whether they're in insert or normal mode without relying on other indicators (statusline, etc.).