Let's add a new COMPASS_LG_DARKMODE feature flag (off by default).

When the COMPASS_LG_DARKMODE is enabled the components in compass-components that use leafygreen should pass the darkMode=true to leafygreen components. It may make sense to create a High Order Component (HOC) for this. Additionally, when the feature flag is enabled, the `darkreader` should not enable itself:
https://github.com/mongodb-js/compass/blob/main/packages/compass/src/app/theme.js#L15

Note: We currently have an override for the leafygreen toggle that uses leafygreen's darkmode already. We should preserve that with and without the feature flag.