Match GitHub fonts for pretty print

[isoos]

The current font makes certain table lines too long.

• GitHub: https://github.com/desktop-dart/text_table
• Dartdoc: https://www.dartdocs.org/documentation/text_table/2.5.3/

GitHub uses font-family: "SFMono-Regular", Consolas, "Liberation Mono", Menlo, Courier, monospace; which the pub site will match. I'm happy to create a PR here too, just wanted to check if there is any preference of one font over the other.

[jcollins-g]

I have no preference; please go ahead and file a PR preferably with screenshot(s).