feat: Add focus on initial render to Form Error List

[alimpens]

This adds focus on initial render to Form Error List, as prescribed by gov.uk.

I had to split the component in 2 parts. One part sets the document title, creates a state and returns early if there are no errors.
The second part is what renders when the component does receive a list of errors.
These parts had to be separated because the early return caused the hooks used (useRef, useImparitiveHandle, useEffect) to be called conditionally, which isn't allowed by the rules of hooks.

The hasFocusedOnce state is necessary to prevent the component receiving focus every time it's shown on screen. React Hook Form validates on submit and revalidates on change, which causes the component to appear while your typing. You don't want to move focus from the input you're typing in to the Form Error List in those cases.