Add drop into editor API

[mjbvz]

Exploration of dragging a file from VS Code's explorer into an editor to create a link. Example use case would be dragging a file into a markdown editor to insert a link

[mjbvz]

I have a prototype but need to think more about what the design should look like. Biggest question I have right now:

Should we allow there to be multiple of these drop handlers active at one time? We do want to allow there to be multiple drop handlers where each one handles a different dropped mime type, but what about multiple handlers for a single mime type? In markdown for example, you can imagine having one drop handler that creates links and another that creates image links

If we did allow this, presumably we then need some way for the user to control what happens on drop, either with a setting to configure the default or by presenting a list of actions to them

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Answering this helps determines if we go with more of a provider style api or more of an event api