feat: support on-event command for watch changes

[adminy]

This originated from feature request described here: https://github.com/fastify/fastify-cli/issues/569

This feature addition adds a watch command parameter that you can run every time there is a watch event change.

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

[adminy]

fixed the extra new line linting issue.

[mcollina]

CI is failing