fix: MergeDeep type to more gracefully merge optional nested object types

[mesqueeb]

type D1 = { [key in string]?: { cool: boolean } | null }
type D2 = { [key in string]?: { notCool: boolean } | null }
type TestD = PrettyPrint<MergeDeep<D1, D2>>

current

{
    [x: string]: {
        cool: boolean;
    } | {
        notCool: boolean;
    } | {
        cool: boolean;
        notCool: boolean;
    } | null;
}

expected

{
    [x: string]: {
        cool?: boolean;
        notCool?: boolean;
    } | null;
}