Add typing utility module

Add a typing utility module with a helper function to "cast" the input
so that TS understands that it is not null/undefined. This is useful
in situations where logic would prevent a reference from being empty
but TS can't follow that logic.

src/sidebar/util/typing.js

+/**
+ * Helper function for cases in which logically a reference is definitely
+ * not nullish, but TS can't infer that correctly. This will cast the `arg`
+ * and appease type-checking.
+ *
+ * @template T
+ * @param {T} arg
+ */
+export function notNull(arg) {
+  return /** @type {NonNullable<T>} */ (arg);
+}