Line numbers for code blocks

[sepsol]

Is your feature request related to a problem? Please describe.
It is really hard to refer to parts of code blocks in discussions when they don't have line numbers.

Describe the solution you'd like
An option to allow users to enable line numbers in code blocks they send.

Describe alternatives you've considered
Slack already had this feature. In Slack, if you press Ctrl + Shift + Enter you can have code blocks (files) that have line numbers.

Additional context
How it works in Slack: