Assert Exit this page HTMLAnchorElement button and clearly guard all dynamic fields

[colinrotherham]

This PR is a follow up to #4129 (see #4199) to fix the "link button" check:

- if (!($button instanceof HTMLElement)) {
+ if (!($button instanceof HTMLAnchorElement)) {

We should catch this early to ensure location.href changes work correctly

- window.location.href = this.$button.getAttribute('href')
+ window.location.href = this.$button.href

I've also added some extra guards for dynamic elements so it's clear which ones we chose to throw errors for

[github-actions]

📋 Stats

File sizes

File	Size
dist/govuk-frontend-4.7.0.min.css	120.66 KiB
dist/govuk-frontend-4.7.0.min.js	51.56 KiB
dist/govuk-frontend-ie8-4.7.0.min.css	81.59 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	75.78 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	71.15 KiB
packages/govuk-frontend/dist/govuk/all.mjs	3.84 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	359 B
packages/govuk-frontend/dist/govuk/govuk-frontend.min.css	113.99 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	36.87 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.3 KiB

Modules

File	Size
all.mjs	67.48 KiB
components/accordion/accordion.mjs	20.61 KiB
components/button/button.mjs	4.55 KiB
components/character-count/character-count.mjs	20.73 KiB
components/checkboxes/checkboxes.mjs	5.69 KiB
components/error-summary/error-summary.mjs	5.85 KiB
components/exit-this-page/exit-this-page.mjs	15.81 KiB
components/header/header.mjs	3.84 KiB
components/notification-banner/notification-banner.mjs	4.39 KiB
components/radios/radios.mjs	4.68 KiB
components/skip-link/skip-link.mjs	3.73 KiB
components/tabs/tabs.mjs	9.41 KiB

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Action run for 8d85f0a

[domoscargin]

Looks good to me, though I've had to get my head out of the Error messages space to allow it to accept early returns again 😆

Could we sneak in a coupla words to cover the guards in the PR title? Or give it a more general message about tightening up checks?

[colinrotherham]

@domoscargin Thanks for reviewing, all updated and rebased

[domoscargin]

🚀