WebGPU.js: Added GPUShaderStage polyfill.

examples/jsm/capabilities/WebGPU.js

@@ -1,3 +1,9 @@
+if ( window.GPUShaderStage === undefined ) {
+
+	window.GPUShaderStage = { VERTEX: 1, FRAGMENT: 2, COMPUTE: 4 };
+
+}
+
 class WebGPU {
 
 	static isAvailable() {