Add -IgnoreOrder to Should-ContainCollection

[nohwnd]

Fix #2874

Should-ContainCollection requires the expected items in the same order, which is annoying when migrating from Should -BeIn where the order did not matter. This adds an -IgnoreOrder switch that only checks that the expected items are present, in any order.

'b',3,2,'a',$null | Should-ContainCollection 'a',2,$null -IgnoreOrder

The check is a multiset subset (new internal Is-CollectionSubset), so duplicates and $null are handled correctly, each expected item is matched at most once. Without the switch the ordered behavior is unchanged.

Added tests for the ordered and unordered cases, duplicates and $null.

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