New feature: copy / reimplement errorSummary component javascript

The errorSummary component javascript is not included or reimplemented in this library.

The javascript is needed to correctly focus the errorSummary when shown, and to handle correctly scrolling the page when the error links are used.

I think this library will need to reimplement the js in cases where the library is used client side.

This is closed by #267 as we now use the ErrorSummary class from NHS.UK frontend

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