Improvements to the Markdown logic and rendering

[hjiangsu]

Pull Request Description

This PR introduces some cleanup and logic improvements to the Markdown widget:

• Extracted the Lemmy community/user markdown, spoiler markdown, and super/subscript markdown logic into dedicated files
• Simplified the logic within CommonMarkdownBody to hopefully improve performance

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?