[context] improve naming of dispose and multiple arguments

[benjamind]

Currently the proposal outlines that the context-event should carry a payload that can include the optional argument multiple in order to indicate that the consumer will handle multiple repeated deliveries of the context value. The callback in the payload then receives an optional second argument dispose which is a function which can be invoked by a component to indicate to its provider that it no longer wishes to receive updates.

I'd like to propose that we rename these two parameters to make them more related to each other:

export type ContextCallback<ValueType> = (
  value: ValueType,
  unsubscribe?: () => void
) => void;

export class ContextEvent<T extends ContextKey<unknown>> extends Event {
  public constructor(
    public readonly context: T,
    public readonly callback: ContextCallback<ContextType<T>>,
    public readonly subscribe?: boolean
  ) {
    super('context-request', {bubbles: true, composed: true});
  }
}

So an event is emitted with an argument in its payload to subscribe to further updates to the context value, and then receives in its callback a second argument to unsubscribe from those updates.

This feels to me like it clarifies the intent and behavior here.

[justinfagnani]

I like subscribe and unsubscribe(). I think it makes it more clear when the callback should be retained by providers, so the answer to #24 is more obvious.

I can make a PR for this.