Use explicit .js extensions in relative imports

Consumers using moduleResolution NodeNext need explicit extensions in
the emitted declaration files. Covers every source file except app.ts,
app-bridge.ts and events.ts (and their tests), which are being reworked
separately.

src/events.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it, spyOn } from "bun:test";
 
-import { EventDispatcher } from "./events";
+import { EventDispatcher } from "./events.js";
 
 type TestEventMap = {
   change: { value: number };