Add URL handling for opening the multidiff editor for a PR

[alexr00]

We have the uriHandler.ts. We should add to it to have handle a new path /open-pull-request-changes, which opens the multidiff editor for a PR.

[alexr00]

To verify:

• Construct a URI for a PR like this:
  vscode-insiders://github.vscode-pull-request-github/open-pull-request-changes?uri=https://github.com/microsoft/vscode-css-languageservice/pull/460
• Open it
• Verify that the multi diff editor for the PR you referenced in your URI is opened.