Use the new `value` feature in Frontend

[joelanman]

Context

GOV.UK Frontend has a new feature to set values in components. We should update the kit/docs (the checked function is covered in the tutorial)

Alternatives

Currently the kit has a function called checked that achieves the same thing. Maybe we can retire it in favour of values

[36degrees]

Related proof of concept I put together previously – https://github.com/alphagov/govuk-prototype-kit/pull/1316

I think all form components except date input could be extended in the same way to pre-populate values from the data object.