[nice to have] Would be very nice to have a `BeAnArray` matcher

[eriknyk]

Would be very nice to have a BeAnArray matcher since we have similar Matchers like

.Should(BeADirectory())
.Should(BeARegularFile())
.Should(BeEmpty())

Would be Nice

.Should(BeAnArray(...))

This can be helpful when a variable has type any = interface{} it can be help to identify if the variable type is an expected array

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Best Regards.

[onsi]

Hey there - happy to accept a PR for this if you have the bandwidth to put one together.