Implement mapReadAsync function of GPUBuffer

Implemented the `mapReadAsync` and fixed the `unmap` functions of `GPUBuffer`.
Added `mapped` internal slot for tracking the ArrayBuffer/Promise.
Added more states to the `GPUBufferState` enum.

components/script/dom/bindings/codegen/Bindings.conf

@@ -150,6 +150,10 @@ DOMInterfaces = {
 
 'GPUAdapter': {
     'inRealms': ['RequestDevice'],
+},
+
+'GPUBuffer': {
+    'inRealms': ['MapReadAsync'],
 }
 
 }