feat: support group fields in slices (#62)

* chore: update `@prismicio/client`

* feat: support groups in fields

* ci: temporarily install deps using `--legacy-peer-deps`

* fix: add group item interface to output

* test: remove `only`

* chore: update @prismicio/client

* chore: update packages

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Co-authored-by: Xavier Rutayisire <[email protected]>