feat: allow spy warning and return emitted state

[climba03003]

Fixes fastify/fastify#6878

1. Return the current emitted state when warning is called.
2. Allows spy the warning

No performance is dropped for adding the spy feature.

# Feature Branch
process-warning> node .\benchmarks\warn.js
warn x 188,557,324 ops/sec ±3.67% (89 runs sampled)
(node:36000) [TST_ERROR_CODE_1] TestWarning: message
(Use `node --trace-warnings ...` to show where the warning was created)
(node:36000) [TST_ERROR_CODE_2] TestWarning: message

# Main Branch
process-warning> node .\benchmarks\warn.js
warn x 185,718,346 ops/sec ±3.11% (86 runs sampled)
(node:39960) [TST_ERROR_CODE_1] TestWarning: message
(Use `node --trace-warnings ...` to show where the warning was created)
(node:39960) [TST_ERROR_CODE_2] TestWarning: message

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Tony133]

lgtm

[jean-michelet]

I would try to improve the English a little bit for the doc.
@Fdawgs?

[climba03003]

Document (it's developer) and English (non native language) really not my strength.
Welcome for improvement.

[jean-michelet]

Not my strength either, this is why I pinged FDawgs