TS: Add `isWebGL2` for WebGLCapabilities.

src/renderers/webgl/WebGLCapabilities.d.ts

@@ -11,6 +11,7 @@ export class WebGLCapabilities {
 		parameters: WebGLCapabilitiesParameters
 	);
 
+	isWebGL2: boolean;
 	precision: any;
 	logarithmicDepthBuffer: any;
 	maxTextures: any;