Add difficulty adjustment examples.

examples/nodejs/bitcoin/difficulty.ts

@@ -0,0 +1,11 @@
+import mempoolJS from "@mempool/mempool.js";
+
+const init = async () => {
+  const {
+    bitcoin: { difficulty },
+  } = mempoolJS();
+
+  const difficultyAdjustment = await difficulty.getDifficultyAdjustment();
+  console.log(difficultyAdjustment);
+};
+init();