feat: better async transform (yield replace await)

[paoloricciuti]

So after today discussion i couldn't bother not trying this.

I've passed back the value from the awaited next if the function is a generator allowing for a syntax like this

task(async function*(){
    const value = yield fetch("something");
});

this still sucks in TS so we should still recommend users to do this

task(async function*(){
    const value = await fetch("something");
    yield;
});

BUT

this allow us to change the async transform to transform this

task(async() => {
    const value = await fetch("something");
});

into this

task(async function*(){
    const value = yield fetch("something");
});

the advantage of this is that if the task is aborted while the promise still hasn't resolved after it resolve we don't assign to value which could be much better if value was a reactive variable in svelte.

In this PR i've also updated the vite plugin to apply this better transform (you can see how the transform change in the generated test files)

[nickschot]

Excellent! :D