Create a whitelist for id-length rule

Hello and thank you for your work.

The `id-length` rule is great for forbidding one-letter variable names. However, there are cases where one-letter symbols would actually be preferred: type parameters (`GenericTypeAnnotation` nodes) and holes.

#### Holes

```js
Array
.from({ length: 10 })
.map((_, index) => index);
```

#### Type parameters

```ts
function unique(collection: T[]): T[] {
return [...new Set(collection)];
}
```

What do you think about passing a config that would allow them?

```json
"id-length": [true, {
"check-type-parameters": false,
"allow": ["_"]
}]
```