Watch commands parameter => run command on watch triggered

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the feature has not already been requested

### 🚀 Feature Proposal

Every time there is a file change I would like to run the standard command on the code, unfortunately there is no arguments that watch takes in to configure it to do that, but it seems like a very useful feature if we could do something pre/post watch event like spotting any linting issues and reporting them in the terminal.

### Motivation

It seems like something a watch command should be able to do, since its not just a mechanism for hot server reload but sometimes it can be extra, like running a linter command.

### Example

```bash
fastify start index.js -wPV --ignore-watch='node_modules .git .vscode' --on-watch-change='standard --fix'
```