Feature request: flare handler that inspects REPL response and triggers behavior including display of a Webview

[timothypratley]

Feature:

When receiving the results of a REPL evaluation, Calva will inspect the value to determine if it is a "flare". A flare is a special value that is unmistakable a request for Calva to do something, like trigger an Information Message in the IDE, or show a Webview. A more detailed explanation is offered in this 5 min video about flares

Motivation:

1. Visualization tools like Clay would benefit from being able to display html in a panel instead of an external browser.
2. More broadly there is a common need for code running on the REPL server to request behavior in the REPL client (Calva).
3. Triggering IDE behavior from code opens up new automation and integrated experiences.

Prior Art:

Cursive recently introduced inline-nodes. Fundamentally they are the same idea, however it is worth noting that Cursive goes further down the path of enabling users to construct complex UI elements with nesting.

Proof of concept:

https://github.com/timothypratley/clayva