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/inspector/mod.rs

@@ -6,7 +6,11 @@ mod wasm;
 pub struct InspectorPlugin;
 
 impl Plugin for InspectorPlugin {
-    fn build(&self, app: &mut App) {
+    fn build(
+        &self,
+        #[cfg(feature = "wasm")] app: &mut App,
+        #[cfg(not(feature = "wasm"))] _: &mut App,
+    ) {
         #[cfg(feature = "wasm")]
         app.add_systems(
             PostUpdate,