Throw in Add-ShouldOperator when max number of operators is reached

[fflaten]

PR Summary

Throws an exception when attempting to add a new operator when there's already 32 registered.

PowerShell only allows 32 parameter sets due to a uint-bitmask used to tag which sets a parameter is included in. This won't be fixed in Windows PowerShell and unfortunately PowerShell doesn't throw an exception due to a bug.

When the 33rd (or more) operators are added using Add-ShouldOperator it overflows and won't work. The new operator name will use a built-in behavior. Pester currently includes 26 operators, leaving only 6 functional slots for custom assertions per session.

Related #1355 Issue won't be fixed until operators are used without Should parameter sets.

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