Add webview for REPL output

[bpringe]

The idea behind this output location is for it to be a read-only webview. The initial release will likely not come with any additional mechanism for input beyond using comment forms in files, which is fine by me, because I never type / eval code in the output window.

If desired in the future, we can add an additional input mechanism, which can even live inside the webview, but would be separate from the output area. Mixing input and output in the same container adds complexity that we want to avoid going forward.