Variables View: Provide UI for evaluating Promises

[kevinushey]

R has objects called "promises", which are effectively objects which are evaluated only on first use. In RStudio, we display them like so:

We also allow users to click on a promise to "force" it, and replace it with the underlying evaluated value. We may want to consider a similar feature for Positron.

[jmcphers]

@kevinushey pointed out on Slack ( https://positpbc.slack.com/archives/C04FPQK3H9C/p1681316371106569 ) that active bindings are similar -- we don't want to trigger their evaluation implicitly, but also want users to be able to trigger them explicitly when they need/want to see the underlying value.

I think we can generalize this a little to avoid having the Environment pane become too knowledgable about R.

• A variable can list a set of actions that can be performed on it; each action could have a name and optionally a codicon name
• When we show the variable in the Environment pane, we make the actions it advertises visible somewhere (context menu and/or via icons drawn directly on the row)
• Invoking the action from the UI would cause a message to be sent to the environment comm with the variable path and the desired action.