- Basing snippet insertion failure on a new `_codeEditor` null-check.
- Now returns `Thenable<boolean>`.
- Removed vscode.proposed.d.ts copy of the `TextEditor` change.
- Removing empty options interface.

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.

* a declared module allows to have a sibling module containing experimental API
* this change also cleans up some bogous type pollution that happened because vscode used to be both, a declare module and a global namespace

This reverts commit 2d1c94ab.

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