QOL: Move source code under the src directory. (#1318)

src/utils/testing.ts

@@ -0,0 +1,3 @@
+export const isTesting = () => process.env.NODE_ENV === 'test';
+
+export default { isTesting };

src/utils/type-utils.ts

@@ -0,0 +1,5 @@
+export type Entries<T> = {
+  [K in keyof T]: [K, T[K]];
+}[keyof T][];
+
+export type ValueOf<T> = T[keyof T];