In TS code, convert Shiny from interface to class

[wch]

Some issues we currently have with the Shiny interface in our Typescript code:

• Some function types end up being circular if you Cmd-click on them, like shinyUnbindAll.
• Some of the methods on the Shiny object appear only after initialization, like setInputValue, bindAll, user, and so some authors might wait for those methods to be available, which is weird.

  shiny/srcts/src/shiny/initedMethods.ts

  Lines 11 to 22 in e2b7f91

  	type FullShinyDef = Required<
  	Pick<
  	Shiny,
  	| "bindAll"
  	| "forgetLastInputValue"
  	| "initializeInputs"
  	| "oncustommessage"
  	| "setInputValue"
  	| "shinyapp"
  	| "unbindAll"
  	| "user"
  	>

• It's just weird to have a Shiny interface and a FullShinyDef type.
• It's strange that we initialize window.Shiny as an empty object, but we say that it is a Shiny object.

  shiny/srcts/src/window/libraries.ts

  Lines 3 to 10 in e2b7f91

  	function windowShiny(): Shiny {
  	// Use `any` type as we know what we are doing is _dangerous_
  	// Immediately init shiny on the window
  	if (!(window as any)["Shiny"]) {
  	(window as any)["Shiny"] = {};
  	}
  	return (window as any)["Shiny"];
  	}

Proposed solution (with help from @schloerke):

• Change Shiny from an interface to a class, and make sure it has all methods from the beginning.
• Add an init() method to that class.
• If some of the methods are called before init(), then throw an error, with information on how to fix the problem.
• Add initPromise() method, which returns a promise that resolves at the end of init(). Then code that needs to wait for Shiny to be ready can just wait for that.
• Add an onInit() method, which lets you register a callback to be invoked when the initPromise() resolves.