PR #5779 Modular, multi-threaded, transitioning plugins

Several improvements have been made to the plugin system:

• There’s no longer a dedicated plugin type. plugin(...) now directly returns a module. This means we can use import with plugins and .with with plugin functions.
• Plugins now support multi-threading automatically, requiring no manual intervention from users.
• A new plugin.transition API has been introduced for executing non-pure functions. Calling non-pure functions without using transition may lead to unpredictable results.

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#import plugin("mycalc.wasm"): myadd, mymut
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#myadd(3, 5) \
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#mymut.with(3)(5)

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#let base = plugin("mut.wasm")
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State before mutation: #base.get() \
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#let mutated = plugin.transition(base.add, "hello")
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State after mutation: #base.get() \
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Returned mutated value: #mutated.get()