Typing `separator` with an array instead of just string

### Prerequisites

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

### Fastify version

4.19.2

### Plugin version

_No response_

### Node.js version

20.x

### Operating system

Linux

### Operating system version (i.e. 20.04, 11.3, 10)

6.4.1

### Description

Hello,

I would like to type `ALLOWED_HOSTS` as `string[]` instead of `string`.

```js
interface ConfigEnv {
ALLOWED_HOSTS: string
}

export function loadConfig(): ConfigEnv {
return envSchema({
schema: {
type: 'object',
required: [
'ALLOWED_HOSTS',
],
properties: {
ALLOWED_HOSTS: {
type: 'string',
separator: ',',
},
},
},
})
}
```

Now the actual type of `loadConfig().ALLOWED_HOSTS` is an array, empty or not. Here the type is incorrect to be `string` and not `string[]`. But if I change it I get the error on the line of `type: "string"`:

```Type '"string"' is not assignable to type '"array"'.ts(2322)```

How can I do? Thanks.

### Steps to Reproduce

```js
// a.js
import envSchema = require("env-schema")

interface ConfigEnv {
ALLOWED_HOSTS: string[]
}

export function loadConfig(): ConfigEnv {
return envSchema({
schema: {
type: 'object',
required: [
'ALLOWED_HOSTS',
],
properties: {
ALLOWED_HOSTS: {
type: 'string',
separator: ',',
},
},
},
})
}

// tsconfig.json
{
"compilerOptions": {
"target": "ES2020",
"module": "commonjs",
"lib": [
"es2020",
"dom"
],
"declaration": true,
"strict": true,
"noImplicitAny": true,
"strictNullChecks": true,
"noImplicitThis": true,
"alwaysStrict": true,
"noUnusedLocals": false,
"noUnusedParameters": false,
"noImplicitReturns": true,
"noFallthroughCasesInSwitch": false,
"inlineSourceMap": true,
"inlineSources": true,
"experimentalDecorators": true,
"strictPropertyInitialization": false,
"typeRoots": [
"./node_modules/@types"
],
"outDir": "dist",
"resolveJsonModule": true
},
"exclude": [
"node_modules",
"cdk.out",
"dist"
]
}
```

Run:

```sh
$ ts-node a.ts
```

Error:

```
TSError: ⨯ Unable to compile TypeScript:
a.ts(16,11): error TS2322: Type '"string"' is not assignable to type '"array"'.
```

### Expected Behavior

Type `ALLOWED_HOSTS` as:

```js
interface ConfigEnv {
ALLOWED_HOSTS: string[]
}
```