refactor(QuickChat): split decoration to hint and gutter icon

Description

This MR is preparation for feat: create quick chat hint when highlighting ... (!2550 - merged)

It refactors the Quick Chat decoration logic in two separate commits:

• The first commit splits the decoration class into two. This commit is not changing any behaviour, only moves relevant code and tests between files
  • gutter icon - this one will stay untouched
  • hint - this will be further refactored
• The second commit changes the interface of the Hint component
  • We dispose the component whenever we don't want to show the hint (instead of passing in "shouldShow" boolean flag)
  • We pass in the full selection change event to the component, which is further used in feat: create quick chat hint when highlighting ... (!2550 - merged)

The new, isolated, Hint component is then rewritten in the next MR.

Related Issues

Part of [VS Code] QuickChat keybinding hint strange beh... (#1729 - closed)

How has this been tested?

1. Start the extension
2. Make sure your Duo Chat is working (e.g. the side panel shows)
3. Put the cursor on an empty line: see the hint
4. Put the cursor on a non-empty line: the hint goes away
5. Start a quick chat
6. See the gutter icon
7. Switch between editors and go back to the one with quick chat
8. See the gutter icon

This is a screencast with the testing:

decoration-refactor

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes