Add a `Chat: Perform Code Review` command

[benvillalobos]

This can help improve discoverability of our local copilot code review functionality.

Commands to add

• Chat: Code review current selection
• Chat: Code review unstaged changes

[benvillalobos]

Turns out this already exists! > Chat: Review will trigger a code review automatically on the current selection. Would be nice to open a quick pick for:

• Current selection
• Current diff
• HEAD against main