feat: Add new mainField `vueSource` to package.json

Both webpack and vite are configurable to look at the mainFields of a
project. We currently expose `src/index.js` under `source`. A lot of
other projects, especially typescript projects do the same.

If we just introduce a new main field (`vueSource`) which currently
noone uses, we can easily configure `vite` and webpack to consume
`@gitlab/ui` from source.