Support extension `UriHandler`s

In a vscode extension you can register a URI handler:
https://code.visualstudio.com/api/references/vscode-api#UriHandler

This UriHandler will get invoked when an external webpage redirects into VS Code with the extension identifier specified.

This works in vscode.dev (thanks to @joaomoreno) where the server sets up a well-known route (vscode.dev/callback) that handles requests that will turn around and invoke registered UriHandlers. Note, this works in tandem with vscode.env.openExternal which will give you the exact Uri to redirect into.

vscode-test-web should have a similar capability and I feel like a lot of the logic can be shared.

Additionally, the Microsoft and GitHub auth extensions use UriHandler for logging in so this means that extensions that use our in-box auth extensions can not be tested in vscode-test-web either.

Workaround

Sideloading the extension on vscode.dev works around this and still allows the author to test their extension on web.

Start with the referenced VS Code UriHandler API, vscode.env.openExternal, and the vscode.dev/callback behavior. Define how vscode-test-web should route external redirects to registered handlers, then verify that web extensions using the Microsoft or GitHub authentication extensions can be tested without sideloading.