Scene Inspector (#11)

This PR adds the bones of a user-facing scene inspector.

### Work Done

To facilitate this work, the Bevy/frontend interface has been expanded to
accommodate two-way communications:

- The Bevy app can now dispatch JavaScript `CustomEvent`s via
  `wasm::dispatch_to_js(...)`
- The frontend application can receive those events by listening for the given
  event type on the `window` object

In its current state, the inspection system works by attaching the `Inspectable`
marker component to any entities which should be exposed to the user. A Bevy
system runs on `Update` to check for newly spawned or despawned entities with
that component, and emits events to notify the frontend. The frontend listens
for those events and updates its model of the scene accordingly, which is then
displayed with a simple tree view.

### Future Enhancements

There are many improvements that must be made to make this system actually
useful, but this PR is being expedited to focus on migrating to Bevy 0.12. A
shortlist of planned updates:

- Identify and expose the "type" of entity (e.g. lights, cameras, meshes,
  skeletons, etc.) via a field on the `Inspectable` component
- Highlight the currently selected entity in the frontend hierarchy
- Ask the Bevy app to update the selection when the user selects an entry in the
  hierarchy
- Add a separate UI panel to inspect and modify the selected entity's components
  via the frontend

studio/src/wasm/mod.rs

@@ -0,0 +1,90 @@
+use std::{cell::RefCell, sync::OnceLock};
+
+use bevy::window::Window;
+use parking_lot::ReentrantMutex;
+use web_sys::{CustomEvent, CustomEventInit, Element};
+
+mod conv;
+
+pub use conv::*;
+
+pub(crate) type WasmMessageBuffer<T> = OnceLock<ReentrantMutex<RefCell<Option<T>>>>;
+
+pub(crate) fn dispatch_to_js(
+    window: &Window,
+    event_type: &str,
+    // NOTE: We use dynamic dispatch here to keep the wasm binary small(-ish)
+    value: &dyn AsJsValue,
+) -> Result<(), String> {
+    let canvas = try_get_canvas(window)?;
+
+    let detail = value.as_js()?;
+    let event = CustomEvent::new_with_event_init_dict(
+        event_type,
+        CustomEventInit::new()
+            .bubbles(true)
+            .cancelable(true)
+            .detail(&detail),
+    )
+    .map_err(|err| format!("{err:?}"))?;
+
+    canvas
+        .dispatch_event(&event)
+        .map_err(|err| format!("{err:?}"))?;
+
+    Ok(())
+}
+
+fn try_get_canvas(window: &Window) -> Result<Element, String> {
+    let canvas_selector = window
+        .canvas
+        .as_ref()
+        .ok_or("Canvas selector not set on Bevy Window".to_string())?;
+
+    let window = web_sys::window().ok_or("web_sys::window() returned None".to_string())?;
+    let document = window
+        .document()
+        .ok_or("window.document() returned None".to_string())?;
+
+    match document.query_selector(canvas_selector) {
+        Ok(Some(canvas)) => Ok(canvas),
+        Ok(None) => Err(format!(
+            "Failed to find element with selector '{canvas_selector}'"
+        )),
+        Err(err) => Err(format!("JavaScript threw an Error: {err:?}")),
+    }
+}
+
+macro_rules! get_buffer {
+    ($buffer:ident) => {
+        $buffer.get_or_init(|| {
+            ::parking_lot::ReentrantMutex::new(::std::cell::RefCell::new(
+                ::core::option::Option::None,
+            ))
+        })
+    };
+}
+
+macro_rules! write_message {
+    ($buffer:ident, $expr:expr) => {{
+        let buffer = crate::wasm::get_buffer!($buffer);
+        if let Some(lock) = buffer.try_lock() {
+            *lock.borrow_mut() = Some($expr);
+        }
+    }};
+}
+
+macro_rules! read_buffer {
+    ($buffer:ident, |$msg:ident| $stmt:stmt) => {{
+        let buffer = crate::wasm::get_buffer!($buffer);
+        if let Some(lock) = buffer.try_lock() {
+            if let Some($msg) = lock.borrow_mut().take() {
+                $stmt
+            }
+        }
+    }};
+}
+
+pub(crate) use get_buffer;
+pub(crate) use read_buffer;
+pub(crate) use write_message;