[Flight] Progressively Enhanced Server Actions (#26774)

This automatically exposes `$$FORM_ACTIONS` on Server References coming
from Flight. So that when they're used in a form action, we can encode
the ID for the server reference as a hidden field or as part of the name
of a button.

If the Server Action is a bound function it can have complex data
associated with it. In this case this additional data is encoded as
additional form fields.

To process a POST on the server there's now a `decodeAction` helper that
can take one of these progressive posts from FormData and give you a
function that is prebound with the correct closure and FormData so that
you can just invoke it.

I updated the fixture which now has a "Server State" that gets
automatically refreshed. This also lets us visualize form fields.
There's no "Action State" here for showing error messages that are not
thrown, that's still up to user space.

scripts/error-codes/codes.json

@@ -465,5 +465,6 @@
   "477": "React Internal Error: processHintChunk is not implemented for Native-Relay. The fact that this method was called means there is a bug in React.",
   "478": "Thenable should have already resolved. This is a bug in React.",
   "479": "Cannot update optimistic state while rendering.",
-  "480": "File/Blob fields are not yet supported in progressive forms. It probably means you are closing over binary data or FormData in a Server Action."
+  "480": "File/Blob fields are not yet supported in progressive forms. It probably means you are closing over binary data or FormData in a Server Action.",
+  "481": "Tried to encode a Server Action from a different instance than the encoder is from. This is a bug in React."
 }