TS 2.3.1 introduced a breaking change by [switching to covariant types for callbacks](https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#covariance-in-callback-parameters). This change tries to fix these compiler errors in the workbench codebase

Changes pinned property to preview
Adds ITextDocumentShowOptions as main-side DTO

Moves column into the options
Removes overloads from the proxy

- 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.

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