Fix variance issues with babel 7

[pnevyk]

In Babel parser v7 the variance is represented as a node instead of a string. Fixes #334. Relates to #336.

I also spotted ObjectTypeSpreadProperty case in the rule, but no tests for it. It perhaps relates to Flow's type extensions:

type One = {| foo: number |};
type Two = {| bar: boolean |};

type Both = {| ...One, ...Two |}

So before this is merged, we should probably define how these spread types behave (it is not mentioned in docs), then validate if they work as we want, and if not, fix them.

My proposal is to have them below normal properties if asc is used, i.e.:

type Foo = {
     z: number,
     ...Bar,
}

[gajus]

🎉 This PR is included in version 2.49.2 🎉

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