feat: Make gitlab-ui tree-shakeable

Currently we are fully rolling up bootstrap-vue and gitlab-ui into the
entry points:

- `@gitlab/ui` (alias for `@gitlab/ui/dist/index.js`)
- `@gitlab/ui/dist/charts.js`
- `@gitlab/ui/dist/utils.js`

This results in a pretty big bundle, index.js is currently 752 KB in
size, because it contains all components except for our charts.

By disabling the bundling in our builds, we can make usage of webpack's
treeshaking capabilities and just import bundles where they are needed.

As a nice side effect, this reduces compilation time substantially.