Make use of ApplyWorkspaceEditParams's label parameter when applying edits

[rcjsuen]

We should provide labels when calling workspace/applyEdit so that clients can use the label parameter to present something meaningful to the user if it wishes.

[rcjsuen]

Probably blocked by microsoft/vscode-languageserver-node#309. I guess an alternative would be to try to mold the JSON request by hand and send it off ourselves instead of using an API.