Add minimal Gizmo Material (#43)

This is a small feature push that adds a simple Shader for the Gizmo.
This shader only takes one value: the color of the object. Very simple
and fast to compile.

This PR is missing:
- Shader Pre Compiling. This is something that I am looking into now,
however I thought it out of scope for this feature.

Just thought I would drop this real fast. Let me know what y'all think!

---------

Co-authored-by: Danny McGee <dannymcgee@gmail.com>

studio/assets/shaders/gizmo.wgsl

@@ -0,0 +1,9 @@
+#import bevy_pbr::forward_io::VertexOutput
+
+@group(1) @binding(0)
+var<uniform> material_color: vec4<f32>;
+
+@fragment
+fn fragment(mesh: VertexOutput) -> @location(0) vec4<f32> {
+    return material_color;
+}