Emit a better warning when default-exporting an inappropriate declaration

[DanielRosenwasser]

Courtesy of @LPGhatguy in #3914.

Instead of going off the rails, we should resiliently parse the following:

export default interface User {
    hi: string;
}

And give an error like

An 'interface' declaration cannot be 'export default'. Consider writing 'export default User;' separately.

[danquirk]

If we don't fix #3914 we should certainly do this. My preference would be to just allow these sorts of declarations.

[DanielRosenwasser]

Agreed, I'd prefer just allowing these sorts of declarations for the sake of consistency.

[clavecoder]

I'm guessing it's not allowed because interfaces and enums are not EcmaScript objects, correct? What's the list of types this fails on? Just interface and enum or are there more?

[Kingwl]

seems already worked?