unused_label: Detect unknown labels

[wilzbach]

Usually using an unknown label results in compile errors and shouldn't happen, but it turns out it does: #466

However, checking that a label is undefined goes a bit over the capabilities of Dscanner.
Maybe a check that looks at the interior of mixins like in https://github.com/dlang-community/D-Scanner/pull/379/files#diff-aa5faf6676eec251edfdfc4d1acf2319R98 could work?

If such an improvement gets added to the unused_labels check, it should be tested with std.math