allow viewing functions side-by-side

[pbiggar]

Developers sometimes want to look at other functions, sometimes just to refer to them, but often to copy code from them.

[pbiggar]

from #3834:

Sometimes it's helpful to compare similar functions side-by-side. Efficient refactoring also benefits from visually comparing functions. Dark currently doesn't provide this ability.

[pbiggar]

From #2806:

When you're working with HTTP endpoints, REPLs, datastores, and cron's, they're all present on the same canvas and you can rearrange them, look at them next to eachother, etc.

For some reason, functions are different in that you can only look at one of them at a time.

This is a pain, because oftentimes when I'm writing a new function I'm basing it on an existing function or thinking about how it'll be used with an existing function, and I want to see them side by side.

So currently I either need to have two Darklang windows open, or just continually swap between them, which causes unnecessary work.

[pbiggar]

Closing as it doesn't apply to darklang-next