Add simple browser extension (#109276)

* Add support for TS's Hierarchical refactorings API

https://github.com/microsoft/TypeScript/pull/41975

* Add simple browser extension

This change adds a new 'simple browser' extension. This extension uses a webview to render webpages directly in VS Code. We plan on using it for optionally previewing local servers in both desktop and codespaces

The browser itself has a number of limitations due to the security around iframes:

- It traps keyboard focus
- We can't detect if a page fails to load
- We can't track the current url of the iframe

* Add experimental alert when the iframe is focused

* Disable events on focus warning

* Hooking up simple browser to opener