Modernize to ESM-native + CJS wrapper (like the other CLDMV libs)

[Shinrai]

polyfillme is currently CommonJS (no "type": "module", CJS source, CJS test/*.test.js). It should move to the CLDMV standard — ESM-native with a thin CJS wrapper — so consumers get native import while require() still works. Non-urgent; tracking follow-up (same modernization as the CJS→ESM issues on gitmulti / wol-proxy / cldmv-node-memcache).

Reference: CLDMV/wisp (the simplest wrapper)

package.json:

{
  "type": "module",
  // no "main"
  "exports": { ".": { "import": "./index.mjs", "require": "./index.cjs" } },
  "files": ["src/", "index.mjs", "index.cjs", "README.md", "LICENSE", "package.json"]
}

ESM source: index.mjs + src/**/*.mjs.

Thin CJS wrapper index.cjs — loads the ESM build via createRequire and re-exports:

"use strict";
const { createRequire } = require("module");
const esm = createRequire(__filename)("./index.mjs");

module.exports = esm.default;
module.exports.default = esm.default;
module.exports.wisp = esm.wisp;       // re-export each named export
module.exports.wispSync = esm.wispSync;

For polyfillme

• Convert the source to ESM (.js → .mjs), add type: "module" + the exports map, and add an index.cjs wrapper re-exporting the public API.
• Preserve the current public API and any side-effect / global-patch semantics — a polyfill package's consumption pattern (side-effect import vs. explicit apply) must not change behavior across the ESM/CJS split.
• Keep the test/* suite green against the new entry points (the in-progress v4 onboarding already wires @cldmv/vitest-runner; convert or dual-target the tests as part of this).

This is packaging modernization only — no functional change intended.