Configure Submitter disabling

[seanpdoyle]

Depends on #1217

Follow-up to #386

While Turbo's support for disabling a Form Submissions <input type="submit"> or <button> element is rooted in Rails UJS, it
degrades (and has always degraded) the accessibility of those
experiences.

To learn more about the risks involved, read the Don't Disable
Submits section of Adrian Roselli's Don't Disable Form Controls
along with the additional resources mentioned therein.

The risk of degrading accessibility is especially true for Morph-enabled
Form Submissions. If a form submission will trigger a morphing Page
Refresh with the submitter focused, it's likely that the focus is
intended to remain on the submitter.

With the current [disabled] behavior, that is not possible without a
bespoke event handler like:

addEventListener("submit", ({ target, submitter }) => {
  if (submitter) {
    target.addEventListener("turbo:submit-start", () => {
      submitter.disabled = false
      submitter.focus()
    }, { once: true })
  }
})

This commit introduces a Turbo.config.forms.submitter object with two
pre-defined keys: "disabled" (the default until we can deprecate it),
and "aria-disabled".

When applications specify either Turbo.config.forms.submitter = "disabled" or Turbo.config.forms.submitter = "aria-disabled", they
will be able to leverage those pre-packed hooks. Otherwise, they can
provide their own object with beforeSubmit(submitter) and
afterSubmit(submitter) functions.

[seanpdoyle]

@afcapel @jorgemanrubia this is inspired by #774, although it not mentioned directly.

[brunoprietog]

This looks great! As a screen reader user I can confirm that this is a great improvement. Hopefully it can become the default for the next major version.