Override FetchOptions from event listeners

[seanpdoyle]

The problem

Prior to this commit, instances of FormSubmission and FetchRequest
referenced in dispatched turbo:submit-start and
turbo:before-fetch-request events were tedious to modify from a host
application.

For example, the FormSubmission instance made available through
event.detail.formSubmission exposed properties that seemed writable.

Unfortunately, any mutations to formSubmission.method or
formSubmission.location from within host application event listeners
(like turbo:before-fetch-request) occur after the underlying
FetchRequest instance is built, which has subtle and not obvious
implications. In practice, modifications to these properties are
unobserved by the underlying FetchRequest instance, and have no effect
on the ensuing fetch call.

Luckily, if host applications are aware of these issues and limitations,
they can work around them by mutating the
event.detail.formSubission.fetchRequest instance.

The proposal

After this commit, the FormSubmission.constructor will infer the
required data (like [method] and [action] values) from its
HTMLFormElement and optional HTMLElement arguments, then use those
values to construct the FetchRequest instance.

After that construction, any changes to the FormSubmission properties
will delegate to the FetchRequest instance.

In tandem with those changes, the FetchRequest will follow a similar
pattern: extract as much information up-front during construction, then
delegate property reads and writes to properties. In most
cases, it will delegate to its fetchOptions: RequestInit instance.

To achieve this behavior, several module-local functions and type
definitions were shuffled around between the FormSubmission and
FetchRequest modules. The order of arguments for the FetchRequest
constructor were also re-arranged to support omitting values that are
made redundant by default arguments.

[seanpdoyle]

@afcapel @kevinmcconnell I've rebased this branch to account for the removal of types. It's ready for review.

[afcapel]

@seanpdoyle looks good, clever refactor! 🙏