fix(core): Have custom errors be created in the right realm (#17050)

Although PR #16366 did not fully revert `deno_core`'s support for
realms, since `JsRealm` still existed after that, it did remove the
`ContextState` struct, originally introduced in #14734. This change made
`js_build_custom_error_cb`, among other properties, a per-runtime
callback, rather than per-realm, which cause a bug where errors thrown
from an op would always be constructed in the main realm, rather than in
the current one.

This change adds back `ContextState` to fix this bug, adds back the
`known_realms` field of `JsRuntimeState` (needed to be able to drop the
callback when snapshotting), and also relands #14750, which adds the
`js_realm_sync_ops` test for this bug that was removed in #16366.

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