[plug-in] Add Tree View and Tree Data Provider APIs

[ashumilova]

The new plugin model should contain the tree view and tree data provider APIs.
Tree View should have :

• tree view definition
• tree view item definition
• tree view events (TreeViewExpansionEvent, TreeViewSelectionChangeEvent, TreeViewVisibilityChangeEvent)

For tree data provider:

• provide ability to register provider for tree data

[vitaliy-guliy]

Everything is done except 'TreeViewVisibilityChangeEvent'. I cannot find it in the VSCode api.

[vitaliy-guliy]

[ Improvement ] Make it possible to render hyperlinks in the Tree node #3304