Fix dropped tags in Diagnostic -> IMarkerData

[rw-access]

When converting diagnostics from the language client DIagnostic from into IMarkerData, the tag field was dropped.

This keeps that field present in the diagnostic.

Here's confirmation of this locally with the Unnecessary tag on all false values, and no diagnostics on true:

[CGNonofr]

Thank you 👍

Can you just squash your 2 commits please?