feat!: Migrate the package to ESM

[mykola-mokhnach]

Summary

• Add "type": "module" and an exports map to package.json; keep main/types for compatibility.
• Explicitly set "module": "NodeNext" / "moduleResolution": "NodeNext" in tsconfig.json.
• Add .js extensions to all relative imports across lib/ and test/ (required by Node ESM resolution).
• Replace the __filename/import.meta.url dual-mode shim in lib/utils/module.ts with a plain fileURLToPath(import.meta.url), now that the package is unambiguously ESM.
• Add esmock as a dev dependency and rework test/unit/webdriveragent.spec.ts, which stubbed lib/utils's getWDAUpgradeTimestamp via Sinon on a namespace import — real ES module namespaces are non-configurable and can't be mutated directly.
• Add --enable-source-maps to the test/e2e-test scripts.

BREAKING CHANGE: Consumers using require('appium-webdriveragent') must switch to import/dynamic import() — the package no longer ships a CommonJS entry point.
BREAKING CHANGE: WebDriverAgent#retrieveDerivedDataPath() has been removed; use #retrieveBuildSettings() instead.

[github-actions]

🎉 This PR is included in version 16.0.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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