wasi: rename __wasi_unstable_reactor_start()

Upstream WASI has renamed __wasi_unstable_reactor_start() to
_initialize(). This commit updates Node's WASI implementation to
reflect that change.

PR-URL: https://github.com/nodejs/node/pull/33073


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gus Caplan <me@gus.host>