Fulfills the pending promise `p` with `value` (0 if omitted). Reactions
attached with promise_then(3) and suspended `await` expressions resume
from the microtask drain — never synchronously from this call, but
still within the same gametick.
If `value` is itself a promise, `p` adopts its eventual state instead
of fulfilling immediately (flattening): `p` stays pending until `value`
settles, then settles the same way. Resolving a promise with itself is
an error when the cycle is a direct `promise_resolve(p, p)` — that
call errors before anything is allocated. When the cycle arrives
indirectly — a `promise_then()` handler returning the very promise its
result settles, or an `async` body returning its own promise — there
is no call to fail, and `p` rejects with `PROMISE_REASON_SELF_RESOLVED`
(`"*promise resolved with itself"`) instead.
It is an error to settle a promise that is already settled — including
one whose fate is already committed to a pending adoption: after
`promise_resolve(p, q)` with `q` still pending, a second
`promise_resolve(p, ...)` or `promise_reject(p, ...)` errors even
though `p` itself has not settled yet.
It is likewise an error to settle a promise returned by an `async`
function — that promise belongs to the function body. See
promise_reject(3).