fix: Update rollup config with single point of entry

Updates the build process such that we can import components more efficiently.

Before:

import Modal from '@gitlab-org/gitlab-ui/dist/components/base/modal';
import ProgressBar from '@gitlab-org/gitlab-ui/dist/components/base/progress_bar';

After:

import { Modal, ProgressBar } from '@gitlab-org/gitlab-ui';

TODO:

In a follow-up MR, we need to add sideEffects: false to package.json to enable tree-shaking with this method. First we need to investigate why there appear to be some polyfills being automatically added to the output as these are introducing side-effects and may interfere with tree-shaking.