Extension: Support for tree-views

[bryphe]

Several extensions add tree-views and/or sidebar panes to the UI, for example:

• foam - Using foambubble/foam with onivim2 #3112
• Mocha Test Explorer
• alefragnani.project-manager - mentioned in Plugin Problem with alefragnani.project-manager #3042

The API is described here: https://code.visualstudio.com/api/extension-guides/tree-view

And the extension protocol that would be need to implemented is here:

• Main: https://github.com/onivim/vscode-exthost/blob/0d6b39803352369daaa97a444ff76352d8452be2/src/vs/workbench/api/common/extHost.protocol.ts#L297
• ExtHost: https://github.com/onivim/vscode-exthost/blob/0d6b39803352369daaa97a444ff76352d8452be2/src/vs/workbench/api/common/extHost.protocol.ts#L1099

Some of the work required would be:

• Stub out the API for the Main mesage handlers
• Stub out the API for the Exthost request and types
• Generalize the navigation between tree-views in the explorer pane and other sidebars
• Figure out the right interface for integrating the explorer pane, and other sidebars, with contributed tree-views

Related #1058