fix(esm): use node: prefix for built-in module imports

Deno's strict ESM loader rejects bare specifiers for Node built-ins.
TypeScript preserves specifiers verbatim, so the source must use the
node: prefix for both CJS and ESM output to work across Node, Deno,
and other strict ESM runtimes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: buffcode

src/NtpTimeSync.ts

@@ -1,6 +1,6 @@
 "use strict";
 
-import * as dgram from "dgram";
+import * as dgram from "node:dgram";
 import { NtpPacket, NtpPacketParser } from "ntp-packet-parser";
 import { NtpTimeResult } from "./NtpTimeResult.js";
 import { RecursivePartial } from "./RecursivePartial.js";