Support running in vscode.dev

[pokey]

In order to make it easier for people to use Talon for development, we'd like to support the command server when VSCode is running in a web browser. The approach is roughly as follows:

• Leverage Rango's RPC to inject a payload into the window object
• Inject a function onto the window object that the command server can use to send a response payload. Could possibly leverage some code Rango used to have for modifying the window object. See also this SO post
• Issue a keypress from Talon to tell the command server to read from the window object
• Read from the window object in the command server extension and run the command like usual
• Send the response back via the injected callback