Handle manually invoked completion #2055
Description
The LSP spec allows users to invoke completion manually without typing anything.
This scenario is very different from what we currently handle because there's no target node. We simply need to show all possible completion items for the current receiver.
We will need to think about how addons can be invoked for this case and how they should enhance the base response. We may need to allow for a nil target intentionally and then define a special event for it.