refactor(amazonq): Refactor main.ts into a re-usable component (aws#7033)

## Problem
- main.ts is hard to parse and there's circular dependency between
everything

## Solution
- refactor it so that all circular references can be resolved by calling
a property on the WebviewUIHandler class

## Notes
- the code is the **exact same** other then I reference properties on
the WebviewUIHandler class directly, instead of calling the top level
variables
- this makes [hybrid
chat](aws#7032) a lot easier,
since it's directly calling the WebviewUIHandler
- these changes were done ontop of
aws@b127c92,
so the latest main.ts changes are already refactored as well

---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).
- License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: jpinkney-aws