Handle @types/styled-components dependency/peerDependency

[penx]

When users install govuk-react, there should be a clear path to installing @types/styled-components so that types work.

Perhaps this is a peerDependency on govuk-react and @govuk-react/lib

I don't think we want it as a dependency, as styled-components is a peerDependency so the version wouldn't be known.

[penx]

Note rollup removed their types dependencies due to global conflicts

rollup/rollup#3395

Note on types dependencies here, but this doesn't apply to types dependencies of peerDependencies

https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html#dependencies

[penx]

As styled-components is a peerDependency, the types should either be a peer dependency or nothing.

Personally I feel that if the peer application is using TypeScript and styled-components, it should be straight forward to install @types/styled-components without being nudged to do so by peerDependencies.

As such I think the main thing to do here is:

• add a note in the installation section of the readme
• see how we get on with that and perhaps add to peerDepdencies in future if there's a clear issue/reason