Unify Copilot's functionality in a single extension

[alexdima]

We are working towards implementing all the Copilot functionality in a single VS Code extension. To this end, we are testing disabling the Copilot Completions extension and using the Copilot Chat extension as the sole provider of all Copilot functionality. We have ported the vast majority of features and the roll-out should feel transparent to everyone.

But, as with all changes, despite our best efforts, there is a chance that we missed something! If you encounter any problems, please report a new issue, and you can go back to the current behaviour by configuring "chat.extensionUnification.enabled": false or unchecking this toggle:

[alexdima]

This is now rolled out 100% in Insiders and will roll out as an experiment during November.