Add CLI option to specify directory or file to watch in watch mode

[gafreax]

This PR introduces a new option to the CLI watch mode that allows users to specify a directory or file to watch for changes. This enhancement addresses issue #787.

The option are --follow-watch for example

fastify start --followWatch plugins/support.js --watch .

Checklist

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

[mcollina]

Can you add a test that actually verifies the added behavior?

Sorry for the long wait - this slipped through the notifications :(.

[gafreax]

Don't worry, I understand. For sure I'll add it as soon as possible. Thanks for your reply!