refactor: make enabled feature flags an object

Summary

This MR changes extension feature flags storage method to an object so we use it to hide commands and menus behind a feature flag.

Example

To check for a flag:

// package.json
{
  "command": "gl.createSnippet",
  "when": "config.gitlab.featureFlags.snippets"
}

To set a feature flag:

// settings.json
"gitlab.featureFlags": {"snippets": true}