build: add editable to dist type

[layday]

Closes #763.

[dimbleby]

thanks! an alias for the three-way literal would be a convenience for callers, though of course we can write our own

[layday]

We could, though I'm not sure what would be a good name for it. I could also see us adding editable to the Distribution set and creating a secondary, internal literal for the CLI. I'm inclined to leave it as is for now.

[dimbleby]

I'm not sure what would be a good name for it.

Haha, well I guess callers will just have to answer that ourselves.

Honestly I think that declining to put a name on this part of the interface is a minor failure, but it's one that users can paper over.