Revisit Positron's modals

[jennybc]

Capturing a recent Slack conversation inspired by #5648. In that PR, at first I used vscode.window.showWarningMessage(), which looked like this, for modal: true vs. false:

This approach was ultimately rejected in favor of using Positron's API for this, in order to accomodate Workbench. Here's the current result with positron.window.showSimpleModalDialogPrompt():

There's definitely room for improvement here and @softwarenerd is game to implement it.

[jennybc]

In the next iteration, it would be good to know for sure if the message can contain Markdown (and it would be really nice if it could).

[juliasilge]

With respect to markdown, we are currently prioritizing compatibility with the rstudioapi package, which expects HTML tags like <b>bold</b> and not markdown.

[jennybc]

Ah, so then I think we should just add this to the docstring for message.