feat: Application prototype (#14)

Creates a prototype user interface for dyna3 in the `app-proto` folder. The interface is dynamically constructed using [Sycamore](https://sycamore.dev).

The prototype includes:

  * An application state model (the `AppState` type)
    * A constraint problem model (the `Assembly` type), used in the application state
  * Two views
    * A 3D rendering of the assembly (the `Display` component)
    * A list of elements and constraints (the `Outline` component)

The following features confirm that the views can reflect and send input to the model:

  * You can select elements by clicking and shift-clicking them in the outline. The selected elements are highlighted in the display.
  * You can add elements using a button above the outline. The new elements appear in the display.

Co-authored-by: Aaron Fenyes <aaron.fenyes@fareycircles.ooo>
Reviewed-on: glen/dyna3#14
Co-authored-by: Vectornaut <vectornaut@nobody@nowhere.net>
Co-committed-by: Vectornaut <vectornaut@nobody@nowhere.net>

app-proto/src/main.rs

@@ -0,0 +1,42 @@
+mod add_remove;
+mod assembly;
+mod display;
+mod engine;
+mod outline;
+
+use rustc_hash::FxHashSet;
+use sycamore::prelude::*;
+
+use add_remove::AddRemove;
+use assembly::Assembly;
+use display::Display;
+use outline::Outline;
+
+#[derive(Clone)]
+struct AppState {
+    assembly: Assembly,
+    selection: Signal<FxHashSet<usize>>
+}
+
+impl AppState {
+    fn new() -> AppState {
+        AppState {
+            assembly: Assembly::new(),
+            selection: create_signal(FxHashSet::default())
+        }
+    }
+}
+
+fn main() {
+    sycamore::render(|| {
+        provide_context(AppState::new());
+        
+        view! {
+            div(id="sidebar") {
+                AddRemove {}
+                Outline {}
+            }
+            Display {}
+        }
+    });
+}