Refactor Studio entry-points (#54)

This PR heavily refactors the entry-point functions in
`studio/src/lib.rs` to simplify and consolidate. Additionally, the
frontend application should now be configurable as expected with any
valid combination of the following URL parameters:

* `objectId` : A pre-bundled library object to load (e.g., "couch.gltf")
* `bvh` : Any resolvable path to a BVH animation with a MocapNET
skeleton (e.g., "mocap/shuffle.bvh")
* `mixamo` : Any resolvable path to a glTF animation with the Mixamo
skeleton (e.g., "mocap/hip-hop-dancing.gltf")
* `sceneImport` : Any resolvable path to an arbitrary glTF scene
(untested, but should theoretically work)
* `scene` : Any resolvable path to a previously saved Storyteller Studio
scene in `*.scn.ron` format
* `skybox` : The ID of a pre-bundled skybox asset. Valid options are:
  - "gum_trees_4k"
  - "kloofenfal_28d_misty_4k"
  - "meadow_4k"
  - "promenade_de_vidy_4k"
  - "scythian_tombs_4k"
  - "test_scene" (this is the default if not specified)
* `mode` : One of "studio" or "viewer" (the default if not specified)

"Validity" rules are as follows:

* `objectId`, `bvh`, `mixamo`, `sceneImport` and `scene` are mutually
exclusive
* `skybox` is not supported for `scene` (the skybox should be saved and
loaded as part of the scene description, though this is not currently
working)
* Both `mode` options are valid for all other parameters

studio/src/main-headless.rs

@@ -1,6 +1,6 @@
 use bevy::render::render_resource::Extent3d;
 use clap::{Args, Parser, Subcommand};
-use studio::{start_headless, HeadlessMode};
+use studio::headless::{start_headless, HeadlessMode};
 
 #[derive(Parser)]
 #[command(name = "StorytellerStudioHeadless")]