feature: link icon on shapes can now be custom

[danielsuh05]

Fixes Issue #1828

I didn't format the option like the issue said (link.icon) because I was a bit confused by compositeReservedKeyword and some of the other things in compile.go. If you want it to be like the issue said instead of how I did it, I can look back again.

[alixander]

1. it should still be circular, like the plain link icon. please use a circular svg as the test example too (github has one)
2. yes, it needs to be link.icon. it allows for further expansion down the road if needed, instead of flattening out into a bunch of new keywords. related: Container Icon in one of the corner #1653 (comment)

[danielsuh05]

So you want something like this?

x: hello {
    link: https://github.com {
        icon: https://github.com/_____.svg
    }
}

[alixander]

@danielsuh05 Yup.

Add a test for that, but also one for the flat structure

x: {
  link: https://github.com
  link.icon: https://github.com/_____.svg
}