chore: Refactor `MarkdigExtensionSetting` deserialization logics

[filzrev]

This PR contains following changes

• Simplify deserialization logics.
• Remove IncludeFields option. (It seems not used by any available markdig options)
• Add JsonCommentHandling.Skip (Previously it works because comment is dropped with text serialization)
• Add simple test to verify behaviors of above changes.
  • Add markdownServiceProperties test case that contains comment.
  • Modify invalid test data (width/height should be string)