feat: enable async start/stop server

[AllanOricil]

Callback using "done" in vitest was deprecated.

They recommend using async/await or add a arrow function that returns a promise. Since async/await is less verbose, making start/stop server async makes sense.

It shouldn't break current api implementation. People can still use done in test runners that have it

BEFORE/CURRENT

beforeEach(function(done){
    helper.startServer(done);
})

afterEach(function (done) {
    helper.unload();
    helper.stopServer(done);
});

AFTER

beforeEach(async function () {
    await helper.startServer();
});

afterEach(async function () {
    helper.unload();
    await helper.stopServer();
});

[knolleary]

Looks good - the core node unit tests all pass with this change in place.

There is a small risk this someone is calling stopServer without passing in a done callback which may now behave slightly differently due to making it async. But on the whole, I think we can consider that low risk (and simple enough to fix).

But we should make this a semver-minor change to 0.4.