Refactor studio-frontend (#19)

This PR does some rudimentary cleanup / reorganizing to the
`studio-frontend` project.

### Changes
* Refactored `studio-frontend` to use the `<sts-canvas-provider/>` and
  `<sts-viewer/>` components from `studio-web`
* Added some CSS layout utilities for positioning overlayed UI elements
* Extracted `<sts-toolbar/>` and `<sts-toolbar-group/>` to their own
  components
* Extracted `<sts-transform-toolbar/>` to its own component

.vscode/extensions.json

@@ -0,0 +1,5 @@
+{
+	"recommendations": [
+		"runem.lit-plugin"
+	]
+}