refactor(runtime): use Extensions

[AaronO]

Refactors runtime/ to use the new JsRuntime Extensions landed in #9800, paves a first path for a more modular runtime/ and streamlines worker init logic (having both web extensions/ and runtime/ init'd declaratively instead of imperatively)

Changes

• Refactor runtime::ops::*::init to return Extensions
• Refactor workers to init runtime ops with extensions
• Move WebWorkerHandle/sender to OpState

Notes

• This refactoring doesn't include the runtime JS in Extensions, that will require more work, to avoid bugs and duplicating logic with snapshots
• For now we depend on runtime/'s JS to be provided via the snapshot, no different than in main
• However I think we should clean this up & decouple it, so we can boot a Worker without depending on that snapshot

[ry]

LGTM