fileNameRoutePrefix

### 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

Plugin could not only get their prefixes from their parent directories, but also from their filename.

The nature of the feature is very similar to the existing `dirNameRoutePrefix`. It could take either boolean or a function checking/parsing given filename.

### Motivation

It would make routes hierarchy much easier to digest and eliminate some code duplicity and hardcoded prefixes.

If I haven't missed the feature, right now, there's no easy way to declare routes prefix without either hardcoding it into the plugin file or to have a directory with an index file for each route.

### Example

Given this structure:
```
├── plugins
│ ├── hooked-plugin
│ │ ├── autohooks.mjs
│ │ ├── routes.js
│ │ └── children
│ │ ├── commonjs.cjs
│ │ ├── module.mjs
│ │ └── typescript.ts
│ ├── single-plugin
│ │ ├── index.js
│ │ └── utils.js
│ ├── more-plugins
│ │ ├── commonjs.cjs
│ │ ├── module.mjs
│ │ └── typescript.ts
│ └── another-plugin.js
├── package.json
└── app.js
```

fastify-autoload with `fileNameRoutePrefix: true, autoHooks: true` would generate these routes and prefix the leaf plugins accordingly:
```
/hooked-plugin/routes
/hooked-plugin/children/commonjs
/hooked-plugin/children/module
/hooked-plugin/children/typescript

/single-plugin
/single-plugin/utils

/more-plugins/commonjs
/more-plugins/module
/more-plugins/typescript

/another-plugin
```