In generator-aspnetcore-spa, use 'yarn' (if available) instead of 'npm' to restore dependencies because it's > 10x faster

templates/package-builder/typings/npm-which/npm-which.d.ts

@@ -0,0 +1,8 @@
+declare module 'npm-which' {
+    interface NpmWhichContext {
+        sync(executableName: string): string;
+    }
+
+    function defaultFunction(rootDir: string) : NpmWhichContext;
+    export = defaultFunction;
+}