[Fizz] Remove assignID mechanism (#22410)

* Remove pushEmpty

This is only used to support the assignID mechanism.

* Remove assignID mechanism

This effectively isn't used anyway because we always insert a dummy tag
into the fallback.

* Emit the template tag with an ID directly in pending boundaries

This ensures that assigning the ID is deterministic since it's done during
writing.

This also avoids emitting it for client rendered boundaries that start as
client rendered since we never need to refer to them.

* Move lazy ID initialization to the core implementation

We never need an ID before we write a pending boundary. This also ensures
that ID generation is deterministic by moving it to the write phase.

* Simplify the inserted scripts

We can assume that there are no text nodes before the template tag so this
simplifies the script that finds the comment node. It should be the direct
previous child.

Author: sebmarkbage