Refactor entry-point functions (#26)

This PR refactors the `studio` project's entry-point functions for
better maintainability and ease of use for local development.

### Overview
* `start_internal` refactored to return a `bevy::app::App` instead of
  calling `App::run`, allowing for finer-grained configuration of unique
  use cases
* Temporary `start` function added for WASM
* `init` function removed
* `main` standalone entry-point refactored to call `start_internal`

### Fixes
* Crash on non-WASM builds due to missing `StudioMode` resource
* Warning on non-WASM builds due to an unused variable

studio/src/main.rs

@@ -1,3 +1,19 @@
+use bevy::{app::Update, window::close_on_esc};
+use studio::{StudioMode, ViewportSize};
+
 fn main() {
-    studio::init(Default::default());
+    // Edit the arguments here to change the default startup scene or skybox
+    let mut app = studio::start_internal(
+        "sample-room",
+        "gum_trees_4k",
+        "", // ignored for non-WASM
+        "Storyteller Studio",
+        ViewportSize::default(),
+        StudioMode::Editor,
+    );
+
+    // Add any additional systems, resources, etc. for local debugging
+    app.add_systems(Update, close_on_esc);
+
+    app.run();
 }