Embed Widget - additional feature flags to enable UI

[bmingles]

One of the feedback comments we've received for the vscode-deephaven extension is that it is missing a REPL feature deephaven/vscode-deephaven#8). One way we could get this feature "for free" would be to enable the console (or the input portion of it) via a query string argument in Embed Widget. We should think through any existing UI features that would be useful to expose in Embed Widget via query string flags or possibly via the postMessage apis.