feat(bolt11): create getAmount function

src/utils/bolt11.ts

@@ -0,0 +1,15 @@
+import bolt11 from 'bolt11';
+
+/** Decodes the invoice and returns the amount in millisatoshis */
+function getAmount(invoice: string | undefined): string | undefined {
+  if (!invoice) return;
+
+  try {
+    const decoded = bolt11.decode(invoice);
+    return decoded?.millisatoshis ?? undefined;
+  } catch {
+    return;
+  }
+}
+
+export { getAmount };