Code that throws System.Diagnostics.UnreachableException should not be counted as not covered.

.NET 7 introduced the `UnreachableException`, which is basically meant to never been thrown. It is often used in switch statements where a default case is required by the compiler, but most of the time shouldn't be reached.
Coverlet shows the default case as uncovered, but it would be nice to not count this as not covered, since as the exception says the code can't normally be reached.