fix: escape markdown in link labels only

[rileychh]

This PR correctly escapes markdown characters in link labels to prevent Discord from interpreting formatting characters. This is a fixed version of #69, which was closed accidentally and had a bug where it escaped the entire link string (including URLs and markdown syntax) instead of just the labels.

Changes:

• Added markdown_link() helper function that escapes only the label text
• Applied escaping in the render() method at link creation time

Example: A username like @_Mochamoru_ in the author label now displays correctly as plain text instead of rendering with italics due to the underscores.

[rileychh]

Update: Escaping Markdown doesn't work correctly in links in Discord. The same text renders correctly in GitHub. This should not be merged until Discord can correctly render Markdown links.

\_ 
[\_](https://g.co) 
[_italic_](https://g.co) 
[\_escapedItalic_](https://g.co) 
[\_fullyEscapedItalic\_](https://g.co)

_
_
italic
_escapedItalic_
_fullyEscapedItalic_

[Kyrela]

That's what I thought... I actually had this, but removed it in 5025aa0 as it wasn't working as intended, unfortunately.