Fork Form implementation for Pages and App Router (#76498)
The Form component should not need to check at runtime whether it's
running in Pages or App Router — this is a build-time condition.
The implementation is different enough that we might as well fork the
implementations at the module level.
I used #73019 as a reference, since that PR did the same thing for Link.
I have not changed anything in implementation itself; I'll leave that
for subsequent PRs. All I've done is duplicate the existing module, fix
the imports, and update the bundler configuration to resolve to the
appropriate module. There should be no observable change in behavior.
