Bootstrap Image Generator application (#53)

This PR begins scaffolding the Stable Diffusion Turbo application (title
TBD), which is a separate frontend for Storyteller Engine which focuses
on image generation.

### Design Overview

* The application renders a "split-screen" view, with Storyteller Studio
  on the left, and the Stable Diffusion Turbo output on the right
* When changes are made in the Studio editor:
  - A screenshot is taken from the Bevy application
  - Screenshot is piped into Stable Diffusion Turbo model
  - Model output is rendered to the screen
* This all happens in as close to real-time as possible

### Work completed in this PR

* The `studio-frontend` project has been (somewhat crudely) refactored
  to split most of its components into reusable libraries
  - `studio-ui` is a use-case-agnostic design system library for core UI
    components. These are "dumb" components which are controlled
    via `@property` inputs and send `CustomEvent`s in response to user
    input.
  - `studio-frontend` is a collection of reusable Storyteller Studio
    widgets. These are "smart" components which depend on
    `@storyteller/studio` directly, manage their own state, and require very
    little configuration or styling. They are designed to be drop-in
    building blocks that can be reused between various frontends.
* The new application has been bootstrapped as `image-generator-demo`
  - Currently, this simply grabs screenshots from the Bevy application and
    renders those screenshots on the right side of the split-screen view. A
    future PR will wire up to the Storyteller.ai backend to pipe these
    screenshots through Stable Diffusion Turbo, and render that model's
    output instead.

### Testing

* Install dependencies as described in the repo's README
* Run `npx nx serve image-generator-demo`
* Open [localhost:4200](http://localhost:4200) in your browser
  - Use the camera controller in the left side of the view to manipulate
    the Bevy scene
  - Verify the right side of the view updates with a fresh screenshot at a
    rate of ~5 fps
* Toggle the "Realtime Capture" checkbox
  - Verify the right side view no longer updates while the user is
    actively manipulating the editor
  - This should allow the app to run smoother on lower-spec hardware

---------

Co-authored-by: Danny McGee <dannymcgee@gmail.com>

studio-ui/src/lib/button.element.ts

@@ -0,0 +1,32 @@
+import { on } from "@storyteller/framework";
+import { LitElement, html, nothing, unsafeCSS } from "lit";
+import { customElement, property } from "lit/decorators.js";
+
+import styles from "./button.element.scss?inline";
+
+import "./icon.element";
+
+@customElement("sts-button")
+export class ButtonElement extends LitElement {
+	static override styles = unsafeCSS(styles);
+
+	@property() icon?: string;
+
+	override role = "button";
+	override tabIndex = 0;
+
+	@on("keydown")
+	onKeyDown(event: KeyboardEvent) {
+		if (/^( |Enter)$/.test(event.key)) {
+			event.stopPropagation();
+			this.click();
+		}
+	}
+
+	protected override render = () => html`
+		${this.icon
+			? html`<sts-icon icon=${this.icon} aria-hidden="true"></sts-icon>`
+			: nothing}
+		<slot></slot>
+	`
+}