TextEditor.edit snippet overload now returns void. No longer preventing when already in snippet mode.

More robust type validation on ext side of insertSnippet.
Position/range check for snippet insertion was inverted.
Adding insertSnippet to vscode.d.ts. (Should it be in vscode.proposed.d.ts?)
Adding extension API tests for insertSnippet.

fixes #12879

Fixes #8096: Add `options` to TextEditor.edit that allows to control the undo/redo behaviour around the edit