AsyncComputed: can't abort a running computation when no longer needed

[d13]

When using an AsyncComputed in a "lifecycled" resource, I'm unable to abort a running computation.

In my use-case, a web component, it would be handy to have an abort() method that I could call within a component's disconnectedCallback.

It'd probably be brownie points to also expose a [Symbol.dispose]() too for those using the new using keyword in TS 5.2 along with future support for TC39.

I started noodling it on here: main...d13:signal-utils:dispose-async-computed

[justinfagnani]

See https://github.com/lit/lit/pull/3998/files where I added .abort() to lit's Task controller (very similar to AsyncComputed).

A few small differences:

• .abort() takes a reason argument which is passed to the AbortController's abort() method.
• we only abort is the status is pending. I'm not sure if this matters much.

I did want to find a way support aborting by composing AbortSignals, but the only API I came up with was a little awkward: have another constructor option which is a factory that's called before a run to produce a new AbortSignal. Maybe that's fine though.