add wasm.fallback module (#369)

This reverts commit c96e5cf50d.

emscripten/external-wasm.d.ts

@@ -6,6 +6,14 @@ declare module 'external:emscripten/*.wasm' {
     export default wasmPath;
 }
 
+declare module 'external:emscripten/*.wasm.fallback' {
+    /**
+     * This is a wasm fallback url relative from build output chunk.
+     */
+    const wasmFallbackPath: string;
+    export default wasmFallbackPath;
+}
+
 declare module 'external:emscripten/*.js.mem' {
     /**
      * This is a js.mem url relative from build output chunk.

emscripten/spine/spine.d.ts

@@ -6,6 +6,10 @@ declare module 'external:emscripten/spine/spine.wasm.js' {
     export default SpineWasm;
 }
 
+declare module 'external:emscripten/spine/spine.wasm.fallback.js' {
+    export default SpineWasm;
+}
+
 // tslint:disable
 declare function SpineWasm (moduleOptions?: any): Promise<void>;