Add new sleep() function and deprecate resolve() and reject() functions

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This changeset adds a new sleep() function and deprecates the resolve() and reject() functions in favor of the new sleep() function.

The new sleep() function does precisely what the current resolve() function does: It creates a promise that will be fulfilled in a given number of seconds. The only difference is that the sleep() function resolves with no value, whereas the resolve() function resolves with the number of seconds. The previous type was a pretty arbitrary design decision I made, the new type now matches how usleep() returns a void.

Using the name sleep() makes a lot more sense as introducing an asynchronous delay is a rather common requirement and it makes sense to mimic PHP's built-in sleep() function. On top of this, this also helps avoid any ambiguities, as the React\Promise\resolve() function does something entirely different than the React\Promise\Timer\resolve() function (same local name, only different namespace). This is also a major DX improvement, as IDE autocompletion would always suggest the wrong function import, let alone the fact that you wouldn't look for a function named "resolve" if you want to "sleep".

Similarly, the reject() function was mostly added to complement the resolve() function, but I don't see any real-world use cases for this. Implementing this on top of the sleep() function is trivial (see its implementation), so I will argue it provides very little value.

Builds on top of #49

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For the record: I consider the sleep() function to be superior to the old resolve() and reject() functions, but after using this for a while, I think that (re-)using the name "sleep" was not necessarily the best idea. This has definitely been a source of confusion when reviewing code, plus IDE autocompletion may either offer PHP's built-in sleep() or the namespaced React\Promise\Timer\sleep() function, which use different argument types and return values and show different behavior.

I've just filed a new delay() function for reactphp/async in reactphp/async#69 which I believe to be a better name and default behavior considering our plans for ReactPHP v3 as shown in https://github.com/orgs/reactphp/discussions/481. There are plans to deprecate reactphp/promise-stream in the future as per https://github.com/orgs/reactphp/discussions/475 and I wonder if we should perhaps also aim to deprecate reactphp/promise-timer in the future given its limited use and suboptimal API design.