Add events for copy/paste to allow language extensions to bring using/import statements along

[DanTup]

Apologies if this already exists, I can't seem to find anything about it (I expected it'd be listed in complex commands if anywhere).

The language service I use for Dart is getting a new feature that will allow me to send a command to get data about imports/using statements that apply to a subset of code. It's designed to be run when a user copies a chunk of code to the clipboard. When pasting, this data can be exchanged for a bunch of edits that would add the equivalent imports in the new file to avoid the user ending up having to fire a bunch of "import xxx" code actions after pasting.

In order to do this I need to know when the user has copied something into their clipboard (including the file and offset/range) and also again after they have pasted (just the file) so I can add some additional edits.

[DanTup]

It would actually work out better if we could also attach additional data into the clipboard during copy (to avoid there being a mismatch between the import data we stashed and the clipboard, which might have been replaced externally since) and get it back during paste; but I don't know enough about how clipboards work to know how feasible that is!