Migrate memoize module to TS

src/sidebar/util/memoize.js → src/sidebar/util/memoize.ts

@@ -4,21 +4,16 @@
  *
  * The argument to the input function may be of any type and is compared
  * using reference equality.
- *
- * @template Arg
- * @template Result
- * @param {(arg: Arg) => Result} fn
- * @return {(arg: Arg) => Result}
  */
-export function memoize(fn) {
+export function memoize<Arg, Result>(
+  fn: (arg: Arg) => Result
+): (arg: Arg) => Result {
   if (fn.length !== 1) {
     throw new Error('Memoize input must be a function of one argument');
   }
 
-  /** @type {Arg} */
-  let lastArg;
-  /** @type {Result} */
-  let lastResult;
+  let lastArg: Arg;
+  let lastResult: Result;
 
   return function (arg) {
     if (arg === lastArg) {