Provide client library to simplify communication with the renderer

[luttje]

Right now, in order to build a video app some messaging has to occur between the video app and renderer as shown in this example. This communication protocol should happen through an interface. That way consumers do not have to worry about the protocol inevitably changing.

I'm thinking a Typescript package that compiles to a ES module and exports type defintions for TS users.

-outdated psuedo code removed-

[luttje]

I expect that there are conditions where the postMessage sent by the renderer can be delayed and cause a screenshot to be taken too soon. It hasn't happened yet, but there's nothing in the code to suggest it should be synchronous.

Playwright can directly await async evaluated functions before taking the screenshot just fine. We should implement this somehow to improve stability.