Adopt to pre-release extension support

Hi VS Code PM here 👋

Refs: https://github.com/microsoft/vscode/issues/15756

VS Code now supports pre-release extensions. Docs that should help: https://code.visualstudio.com/api/working-with-extensions/publishing-extension#prerelease-extensions

I know VSCode-graphql has a separate Insiders extension. Ideally you would adopt to our pre-release support and we would deprecate that other extension. VS Code can automatically migrate your existing VSCode-graphql Insiders users to the pre-release version of the extension once you have done the adoption.

Let me know what you think