Add VSCode upstream project as a submodule of VSCode Fork

MR: chore: Add vscode upstream as a gitsubmodule (!83 - merged)

Description

As part of transitioning to a patch-based strategy to apply changes to the VSCode upstream project, we want to include the VSCode upstream repository as a submodule of the VSCode Fork repository.

Acceptance criteria

• The VSCode upstream repository is a submodule of the VSCode Repository.
• The submodule ref points to the last version of VSCode released in the VSCode Fork v1.89.1.

Reference

• RFC: Proposal to improve VSCode upgrade process (#32 - closed)
• Use patch strategy instead of force pushing to ... (#5 - closed)