fix: ajvFilePlugin type incompatible with Fastify's ajv.plugins option

## Bug Description

The `ajvFilePlugin` type definition is incompatible with Fastify's `ajv.plugins` option, causing TypeScript errors when registering the plugin.

## Current Type

```typescript
export function ajvFilePlugin (ajv: any): void
```

## Expected Type

```typescript
export function ajvFilePlugin (ajv: Ajv): Ajv
```

## Steps to Reproduce

```typescript
import fastify from 'fastify'
import { fastifyMultipart, ajvFilePlugin } from '@fastify/multipart'

const app = fastify({
ajv: {
plugins: [ajvFilePlugin] // TypeScript error here
}
})

app.register(fastifyMultipart)
```

## Error Message

```
error TS2769: No overload matches this call.
The last overload gave the following error.
Type '(ajv: any) => void' is not assignable to type 'Plugin | [Plugin, unknown]'.
Type '(ajv: any) => void' is not assignable to type 'Plugin'.
Type 'void' is not assignable to type 'Ajv'.
```

## Root Cause

Fastify's `ajv.plugins` option expects `Plugin` type from `@fastify/ajv-compiler`, which is defined as:

```typescript
export interface Plugin {
(ajv: Ajv, options?: Opts): Ajv; // Must return Ajv
[prop: string]: any;
}
```

The actual implementation in `index.js` correctly returns `Ajv`:

```javascript
function ajvFilePlugin (ajv) {
return ajv.addKeyword({...}) // addKeyword returns Ajv
}
```

But the type definition incorrectly declares the return type as `void`.

## Proposed Fix

1. Add `ajv` to devDependencies for type definitions
2. Update `types/index.d.ts`:

```typescript
import type Ajv from 'ajv'

// ...

export function ajvFilePlugin (ajv: Ajv): Ajv
```

## Environment

- `@fastify/multipart`: 9.4.0
- `fastify`: 5.7.4
- TypeScript: 5.8.3