Make type Element[] work with toBeElementsArrayOfSize

[dprevost-LMI]

Fixes #1506. As noted in the issue, when using await + filter on $$(), we get the type Elements[] rather than ElementArray, so we cannot refetch.

This PR ensure that toBeElementsArrayOfSize is compliant with the type `WebdriverIO.Element[], so that the following works.

        const elems = await $$('div').filter(async el => await el.isDisplayed())
        await expect(elems).toBeElementsArrayOfSize(1)

[dprevost-LMI]

Need to merge #1980 first

[christian-bromann]

LGTM 👍

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