Deprecate si prefix

There are some cases where :path is nil on option and we should respect
that.

We already know how to handle `path`, so lets just handle it on our own.

All callers of `action_path` interpolate the return value in to a
string, so there is no need for the method to to_s it.  to_sym on a
symbol will return the same symbol, though I think `action_path` may
always be called with a symbol so this might not be necessary.

we only need to check for `path` once.

[ci skip] Add link to testing guide

Make assert_enqueued_with and assert_performed_with returns the matched job

Now we can see where `defaults` options originate

since `controller` and `controller_scope` were the same, just combine
them

This method isn't used internally, isn't tested, isn't documented.  We
should delete it.

add a predicate method so that we can avoid is_a? calls on the resource
object.

calling `scope` isn't cheap, so try to call cheaper methods that do the
same thing for those particular parameters (in this case `path_scope`)

`resource_scope` should just put resource scopes on the stack, and
doesn't need to know what a `scope_level` is.

We just want to augment the scope level, not the frame itself, so just
copy the frame to the new scope object.

Update info on browser connection limits [ci skip]

Browsers have not been limited to two connections per host for quite
some time now. With more connections allowed, the trade-off involved in
asset host distribution isn't so clear and it becomes more important
that developers measure actual performance before and after.

this gives us an easier way to iterate the stack

The same information is stored in the `@scope` linked list, so just get
it from there.

this lets us remove the setter and make the Resource object Read-Only

We don't need to ask `scope` for the resource because we already have it
right here.

[ci skip] Modify introduction to be consistent with other guides

Closes #21121

Reload I18n.load_path in development

make generated scaffold functional tests work inside API engines

This method raises conditionally not always so we should not documment
as it always raise.

We should return when the contoller key is not present or if the
controller doesn't exist and we didn't raised an error.

Related with dc1b937d

eliminates calling `scope` in one method, pushes the other calls up one
frame.  This goes a little way towards eliminating the internal calls to
`scope`.