chore(core): Split JsRuntimeForSnapshot from JsRuntime (#19308)

This cleans up `JsRuntime` a bit more:

* We no longer print cargo's rerun-if-changed messages in `JsRuntime` --
those are printed elsewhere
* We no longer special case the OwnedIsolate for snapshots. Instead we
make use of an inner object that has the `Drop` impl and allows us to
`std::mem::forget` it if we need to extract the isolate for a snapshot
* The `snapshot` method is only available on `JsRuntimeForSnapshot`, not
`JsRuntime`.
* `OpState` construction is slightly cleaner, though I'd still like to
extract more

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Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>

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