Add shinylive `onUri` handler

[gadenbuie]

Using https://code.visualstudio.com/api/references/activation-events#onUri we could receive URLS like

vscode://Posit.shiny/save?url=https%3A%2F%2Fshinylive.io%2Fpy%2Feditor%2F#code=...

which would let us add an "Open in VS Code" link to the ShinyLive share sheet.