VSCODE-85: Add default button to tree view

[Anemy]

https://jira.mongodb.org/browse/VSCODE-85

This PR adds a button to the tree view for when the user has not yet added a connection. This should improve the experience for users who have just installed the extension.

Note: If the user adds a connection and removes it, the button does not show up. If they refresh their vscode though, and there are no connections, it will show up. When they're first loading the extension and their connections have not yet loaded, the message also shows up - which might be enough of a reason not to use this button format. From what I've read it looks like there's no way for us to specify that our extension is being activated... microsoft/vscode#59645 This will be less of an issue once we're webpacking and load times are hopefully significantly reduced.

Screenshots

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[Anemy]

Added create playground as a dropdown button:

And changed the connect button behavior to opening the connect form instead of connection string input.

[alenakhineika]

Are you sure you want to have create a playground in the same list with connection items in the contextual menu? It looks confusing like you have a third option to connect using a playground. But is is kind of opposite. You can't use a playground if you are not connected.

[Anemy]

@alenakhineika I think it's useful to have access to it in the UI - right now it's a bit tough to discover just through the command palette. I think this is probably more of a temporary solution until we have a proper playground tree view: https://jira.mongodb.org/browse/VSCODE-86