Add static Loop methods

[clue]

This changeset adds static Loop methods. The Loop class now provides all methods that exist on the LoopInterface as static methods:

use React\EventLoop\Loop;

// old (still supported)
Loop::get()->addTimer(1.0, fn () => echo 'Tick');
Loop::get()->run();

// new
Loop::addTimer(1.0, fn () => echo 'Tick');
Loop::run();

This is particularly useful when directly interfacing with the event loop in application code. I've also updated the documentation to highlight some use cases where the Loop::get() method would still be useful when using dependency injection (DI).

Builds on top of #226

[WyriHaximus]

Why are only adding streams marked as advanced? IMHO if we mark those two we should also mark removeReadStream && removeWriteStream as such. And maybe mention react/stream for easier usage.

[clue]

Fair point. I've applied the exact same documentation from the LoopInterface::addReadStream() method:

event-loop/src/LoopInterface.php

Line 8 in 1a709e2

* [Advanced] Register a listener to be notified when a stream is ready to read.

Your suggestion sounds perfectly reasonable, perhaps file this as a follow-up PR to also adjust the interface and the README (similar to #110)? 👍

[WyriHaximus]

Will do 👍

[clue]

Rebased on master now that #230 is in to trigger a new test run

[clue]

Updated to move deprecation notices out of this PR and split this up into a dedicated PR #231. To ease review, this PR here now only deals with the new static methods.

(Merging either PR will likely cause a merge conflict with the other PR, but I'd rather put the burden of rebasing and fixing this conflict onto myself than the people reviewing this.)