Add Racket Support

[smizell]

This PR adds support for Racket, which has some great tools around writing and publishing.

1. Scribble can be used to write any kind of docs. Instead of being a markup language, it's a full programming language.
2. Pollen is a publishing system for web books that can also be printed. Like Scribble, it gives full Racket support in the markup, making it powerful. It also supports Markdown.
3. Frog is a static site generator with blog support.

There are many other writing tools provided by Racket, while Racket also gives the ability to create your own language.

[mraerino]

once we land #411 this formula might be enough for you?
https://formulae.brew.sh/formula-linux/minimal-racket

[smizell]

@mraerino It feels like it might. Would this allow for cached dependencies as well?

[smizell]

Is there anything else you need me to do for this? It looks like it's waiting on a reviewer at the point? Thanks!

[erquhart]

@mraerino it sounds like you're saying that the homebrew package you linked can be used for Racket support, and that this PR is then unnecessary - can you confirm?

[vbrown608]

Thank you for your contribution @smizell. Over the next couple months, we'll be reviewing our use of this build image and how we decide what's preinstalled for builds. Unfortunately, we won't be adding any more dependencies to the build image in the meantime unless there's an urgent reason to do so.

[kitop]

Closing this PR as the it's against xenial branch and the Xenial image is going to be deprecated: https://answers.netlify.com/t/please-read-end-of-support-for-xenial-build-image-everything-you-need-to-know/68239