Add AbortController related helpers

These allow linking multiple AbortControllers and ceate a timeout abort signal.

For example in case you want to abort a fetch in case

(1) the UI element gets removed
(2) a newer request replacing this one gets started
(3) a timeout happens because the fetch takes too long

createLinkedAbortController() allows merging multiple signals into one
and createTimeoutAbortSignal() allows creating a singal that auto aborts
after some time passes.