[VSCode] Add local feature flag for new /fix code actions

Problem to solve

For the implementation of [VS Code] Add /fix to code actions (gitlab-org/editor-extensions&127 - closed) we should ship the feature in multiple small iterations. However users should not see the incomplete / broken feature until it is ready.

Proposed solution

Create a new local feature flag for VSCode, which will hide any parts of the feature while it is in development.

E.g. gitlab.featureFlags.codeActionsDuoFix