Rule request: no-implicit-null-return-types

[JoshuaKGoldberg]

For teams with a large code base, turning on --strictNullChecks can take a while. Having rules that do part of the work would help incrementally enabling it.

If a function may return null or undefined, its return type should reflect this.

Bad:

class Person {
    private name;
    public setName(name: string) {
        this.name = name;
    }
    public getName(): string { // Might be undefined!
        return this.name;
    }
}

Better:

class Person {
    private name;
    public setName(name: string) {
        this.name = name;
    }
    public getName(): string | undefined {
        return this.name;
    }
}

[JoshuaKGoldberg]

This is grander in scale than what a linter can do well. Addressing this would need access to the TypeScript language service.

I've started work on this in TypeStat instead.