chore: add flutter quickstart example, assets and update README

examples/assets/solidcolor.mat

@@ -0,0 +1,14 @@
+material {
+    name : SolidColor,
+    shadingModel : unlit,
+    culling : none,
+    featureLevel : 1,
+    vertexDomain: object
+}
+
+fragment {
+    void material(inout MaterialInputs material) {
+        prepareMaterial(material);
+        material.baseColor = vec4(0.1f, 0.5f, 0.9f, 1.0f);
+    }
+}