add SnippetTextEdit, allow drop-handler to return them

[jrieken]

This helps with #142990. Adds SnippetTextEdit and wires this into the onWillDropOnTextEditor-proposal.

vscode.workspace.onWillDropOnTextEditor(e => {
  e.waitUntil(Promise.resolve(new vscode.SnippetTextEdit(
    new vscode.Range(e.position, e.position),
    new vscode.SnippetString('Hel$0lo ${1|World,Welt,Mondo|}. This is a ${2:snippet}-$2-$2')
  )))
})

Screen.Recording.2022-03-30.at.12.04.06.mov

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Open questions

1. How to prevent multiple snippets from being returned. Provider approach might help
2. Be graceful when an editor isn't available and turn a snippet text edit into a normal edit (no really a problem with dnd into editors thi)

[mjbvz]

Thanks for looking into this!

The new api looks good to me. Once this is merged, I'll clean up the proposed on-drop api