Encode HTTP method into Request body as _method

[seanpdoyle]

Browsers do not handle <form> element submissions with a [method]
value other than get or post. This also applies to <form> elements
submitted with submitter elements that declare [formmethod] values
other than get or post.

To circumvent that constraint, Rails form builders encode other verbs
into a _method parameter that is forwarded to the server.

On the other hand, Turbo's reliance on fetch means that it can
lift [method] or [formmethod] values directly to fetch calls,
regardless of verb.

The way that Rails responds to DELETE or PATCH requests submitted in
that manner differs from those with _method=DELETE or _method=PATCH.
Primarily, controllers that call redirect_to default to using a 302
Found status. When a Turbo request with PATCH receives a 302
response, the subsequent request to the resource is also made with the
PATCH verb, often resulting in unintended side-effects or unhandled
requests.

The Turbo documentation suggests responding to those requests with 303
See Other instead. This is sound advice, and should be applied.

However, for Rails applications migrating to Turbo from Unobtrusive
JavaScript, making that change application-wide can be demanding.

For reasons outside the scope of the pull request, Rails' internal
machinery redirects requests made with POST verbs that encode a
_method parameter in a way that doesn't require a 303.

This commit transforms the existing turbo:submit-start event listener
into a turbo:before-fetch-request listener that transforms requests
with methods other than POST into POST requests that encode their
methods as _method.

The net result is that migrating applications can gradually introduce
303 See Other redirects at their own pace.

[seanpdoyle]

This will need to be ^7.2.0 once a release is cut.