Migrate FormGroup to TypeScript

[colebemis]

This PR migrates the FormGroup component to TypeScript as part of the TypeScript refactor.

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Latest commit: 2565a8e

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[colebemis]

This "component as namespace" pattern is difficult to type with styled-components.

Does have any ideas on how to type this? @smockle @bscofield @VanAnderson

[smockle]

This "component as namespace" pattern is difficult to type with styled-components.
Does have any ideas on how to type this? @smockle @bscofield @VanAnderson

Oooh interesting problem! I do know a few ways we could type this; here are two options:

1. https://github.com/primer/components/blob/30e5c4fc8ea4f4a10eb6cee823ea02e8ea54ac6c/src/FormGroup.tsx
2. https://github.com/primer/components/blob/106012152af520ba72538a454065f098d3cb2ec5/src/FormGroup.tsx

[colebemis]

Found a simple solution to the namespace type issue using Object.assign:

export default Object.assign(FormGroup, {Label: FormGroupLabel})