Port GitHub code display embed from Python

[shenanigansd]

Move the feature that displays the lines of code from GitHub when a link with line numbers is sent from Python to Sir Lancebot

Approved by @Xithrius here: https://discord.com/channels/267624335836053506/635950537262759947/997558518968221696

[Xithrius]

I approve here as well.

[TizzySaurus]

For reference, this is the location of said feature.

[TizzySaurus]

@shenanigansd has stated in #dev-contrib that they're not wishing to implement this themselves, so I'd like to claim this. Can I please be assigned?

[TizzySaurus]

NB: This is now waiting on python-discord/bot-core#141, as lancebot is missing certain util functions.