require-variable-type: Turn off for function expressions

Would be helpful to be able to have a setting such as `excludeFunctionExpressions` or something so that I can write code like this:

```js
const myFunc = function myFunc: boolean {
return true;
};
```

It's just so redundant to have to write something like this when using named function expressions:

```js
const myFunc: Function = function myFunc(): boolean {
return true;
};
```