Export interfaces BasicDropdownSignature, BasicDropdownArgs and BasicDropdownDefaultBlock

[dagroe]

This adds two missing exports for BasicDropdown. These exports help, e.g., to correctly reference and type the Dropdown's blocks in components re-yielding the BasicDropdown blocks.

[dagroe]

@mkszepp Could you review this or maybe ping someone else for review?

[mkszepp]

@dagroe sry, didn't f ound time right now...

I have seen your using in power datepicker and i would suggest following:

Extract this part to an interface, so the code in power datepicker... is more clear and we don't need to access with [0]...

{
        uniqueId: string;
        disabled: boolean;
        isOpen: boolean;
        actions: DropdownActions;
        Trigger: ComponentLike<BasicDropdownTriggerSignature>;
        Content: ComponentLike<BasicDropdownContentSignature>;
}

Could you update this in this PR?

[dagroe]

Hey @mkszepp good idea, thanks. I updated this PR.

[mkszepp]

Thank you, LGTM!