New rule: no-mixed-exports

[MattisAbrahamsson]

This PR adds a new rule "no-mixed-exports"

Mixed exports refers to a file having both a default export and named exports, for example:

export type Options = {
    value: number
}

export default myFunction(parameters: Options) {...}

We regularily comment on this in reviews, so to save us time I wrote this small rule which forbids mixing exports

[Kalle-Rei]

Good stuff. Actually a bit surprised there isn't already a rule for this.

🫡 🇺🇦