Single-object viewer POC (#14)

This commit implements a Sketchfab-style single-object viewer as a minimal
proof-of-concept.

### `@storyteller/studio-web`
This is a new package to house web components for interfacing with the
`@storyteller/studio` WASM library. It currently includes a single component,
`<sts-viewer />`, which is exported from `@storyteller/studio-web/viewer`.

### `react-frontend`
This is a new application intended as a proof-of-concept and testbed for React
interop with `@storyteller/studio-web`.

Web components are currently a little bit fiddly to use in a React application.
`react-frontend/src/app/studio-viewer.tsx` demonstrates how a React-idiomatic
component can be written as a thin wrapper around a web component, and
`react-frontend/src/app/app.tsx` demonstrates that wrapper component in use.

studio/src/interaction/selection.rs

@@ -2,9 +2,12 @@ use bevy::prelude::*;
 use bevy_mod_outline::{
     AutoGenerateOutlineNormalsPlugin, OutlineBundle, OutlinePlugin, OutlineVolume,
 };
-use bevy_mod_picking::{prelude::Pointer, selection::Select};
+use bevy_mod_picking::{prelude::Pointer, selection::Select, PickableBundle};
 
-use crate::input::{self, Action, InputAction, Lifecycle};
+use crate::{
+    input::{self, Action, InputAction, Lifecycle},
+    scene::{SceneElement, SceneSpawnerSystem, SceneState},
+};
 
 pub struct SelectionPlugin;
 impl Plugin for SelectionPlugin {
@@ -14,6 +17,12 @@ impl Plugin for SelectionPlugin {
             .add_systems(
                 PostUpdate,
                 draw_selection_outline.run_if(resource_changed_or_removed::<Selection>()),
+            )
+            .add_systems(
+                OnEnter(SceneState::Active),
+                (apply_deferred, tag_scene_elements)
+                    .chain()
+                    .after(SceneSpawnerSystem),
             );
 
         #[cfg(feature = "wasm")]
@@ -35,6 +44,23 @@ pub struct Selected;
 #[derive(Resource, Deref, DerefMut, Debug)]
 pub struct Selection(Entity);
 
+fn tag_scene_elements(
+    mut cmd: Commands,
+    q_scene_elements: Query<(Entity, &Children), (With<SceneElement>, Without<Selectable>)>,
+    q_primitives: Query<&Handle<Mesh>>,
+) {
+    for (ent, children) in q_scene_elements.iter() {
+        cmd.entity(ent).insert(Selectable);
+
+        for &ent_child in children
+            .iter()
+            .filter(|child| q_primitives.contains(**child))
+        {
+            cmd.entity(ent_child).insert(PickableBundle::default());
+        }
+    }
+}
+
 fn update_selection(
     mut cmd: Commands,
     // FIXME: These events aren't aware of our input actions, so we receive a