Save a Smart Cell for reuse

[josevalim]

By definition we can persist Smart Cells to disk, so it stands to reason we could make prepopulated Smart Cells reusable by saving them. The goal of this feature is to do exactly that: save a Smart Cell for reuse. This may be called "Smart Snippets".

In order for this to work, we will:

• Allow a smart cell to list all its optional dependencies
• Automatically suggest to install any optional dependency at any moment from a smart cell
• Add a button to save a Smart Cell for reuse later. We will give the smart cell a name and we will store how it would be written to disk, alongside its current application and its optional dependencies
• We will have a button in the sidebar where we can manage all snippets
• We will have an option alongside "+ Code + Block + Smart". Either a "Saved" entry inside "+ Smart" or its own icon

You can also save them on Livebook Teams, sharing them with the whole team.

[josevalim]

I was also thinking we can allow people to add to their Livebook something like "Add this smart cell that requires package X and Y with this version", so there are more options in the menu out of the box.

We can also do it for the new files integration: "For this mime-type, use this snippet".

In both cases, it could be part of Livebook Teams, so it is all shared with your team.