Add inline form demo

[daun]

Description

• Add a demo of inline forms: newsletter signup and cart button

Checks

• [x] The PR is submitted to the master branch
• [x] The code was linted before pushing (npm run lint)
• [x] All tests are passing (npm run test)

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[hirasso]

Nice! In the demo's text it says that the keyboard focus is being kept. Is this really the case? The form is being replaced after a click on submit, so the focus will go to the body, won't it?

[daun]

With the newest Forms Plugin update, we're using the id to build a selector, which should make this work. Let me double-check, though!

[daun]

It works. Phew 🥵 The wording is slightly wrong, then — the focus isn't kept but rather restored. For this intro text, I don't think it matters. What do you think?

[hirasso]

Ah ok now I understand! The focus will be moved to/kept on the submitted <form> element, right?

[daun]

Yes! With the current implementation, it will always (re-)focus the form itself, not keep the focus inside the form.