fix: fix misc selection issues

[scottdover]

Summary:
This does a few things around improving context menus and selections.

• It disables the command palette for things that shouldn't work in that way
• It removes context menu items that are being displayed for multiple item selection (although they should only be for single item selection)
• It updates tree view selection to work in the way described in Fix treeview selection #1744

NOTE: The most ideal path here imo is to automatically select an item when issuing the context menu (if it isn't part of the selections). I looked for a way to handle that but was unable. This change seems to work similar to how vscode does things natively.

Testing:

• Made sure things displayed in the command palette worked as expected
• Made sure selection worked as expected