Improved context ergonomics

[TylerBloom]

Currently, the WebTerminal's component Context is passed directly into the TerminalApp methods. While functional, this poses a few ergonomics problems.

First, creating callbacks is much more annoying. The return value of the callback's closure is implicitly converted into the component's message type; however, there is a layer of abstraction between the terminal app's message type and the web terminal message type. This requires the callback creator to either create their message type or construct a web terminal message. Ideally, this same implicit conversion behavior can be maintained.

Second, there is currently no way for the terminal app to access the terminal or the terminal backend. Ideally, this would be accessible via the context.