[PROPOSAL] Provisional features in JupyterLab

[telamonian]

We should adopt a formal process for allowing provisional features in JupyterLab core. These would be new, in-development features that are explicitly marked as such, and would come with the understanding that:

A. The feature will "graduate" to mature within a reasonable timeframe or be removed
B. Until the feature is mature, its API is subject to breaking change

The overarching motivation is to make it easier to develop and get feedback on complex, cross-cutting features that are meant for wider adoption by the community (eg LabIcon, the extension system, anything to do with user settings, etc).

I'll submit a PR with a more complete proposal presently