Single-canvas juggling (#15)

This PR adds a `CanvasProviderElement` to `@storyteller/studio-web` to manage
the complexity of maintaining a single concrete `HTMLCanvasElement` for
`@storyteller/studio`, while still being able to treat components like the
`ViewerElement` as self-contained "views" that behave as expected with regards
to CSS styling, DOM placement, etc.

Consuming applications should wrap the new `<sts-canvas-provider />` element
around some common ancestor of all views into the `@storyteller/studio` app,
similar to a React context provider.

The `react-frontend` application has been updated to take advantage of the new
`CanvasProviderElement` by demonstrating a more realistic use case for the
`ViewerElement`. The new demo presents a grid of "thumbnails," one for each
object in our hard-coded "object library." Clicking on one of these thumbnails
will render a `ViewerElement` for that object in a modal dialog.

utility/src/lib/types.ts

@@ -1,7 +1,23 @@
+export interface Ctor<T, Args extends any[] = any[]> {
+	new (...args: Args): T;
+	prototype: T;
+}
+
+export type AbstractCtor<T, Args extends any[] = any[]>
+	= (abstract new (...args: Args) => T)
+	& { prototype: T }
+	;
+
 export interface Fn<Args extends any[] = [], R = void> {
 	(...args: Args): R;
 }
 
+export interface Method<T, Args extends any[] = any[], R = any> {
+	(this: T, ...args: Args): R;
+}
+
+export type Opt<T> = T | null | undefined;
+
 export type WithOpt<T, K extends keyof T>
 	= Omit<T, K>
 	& Partial<Pick<T, K>>