Update the version of MarkDig

[michaeltlombardi]

Summary of the new feature / enhancement

The version of MarkDig used in the module is 0.22.0. The latest version of MarkDig is 0.30.2.

Because this module is loaded in PowerShell automatically, a non-binary module has no way of using a newer version of MarkDig. There are two particular features added in later releases that would be extremely useful for module authors: the Descendants() method and the ability to round-trip markdown.

Version 0.22.0 was released in October 2020. I suspect that the longer we go between updating the version here, the more work is involved in doing a future update.

Proposed technical implementation details (optional)

Update the version of MarkDig.Signed to the latest version and adjust tests/implementation as required to keep existing functionality.