wasi: update start() behavior to match spec

_start() and _initialize() shouldn't be called from the same
function, as they have different behavior. Furthermore, Node
should throw if both are provided. This commit updates the
implementation, docs, and tests accordingly.

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


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