Return array of matches in `Server::getPlayerByPrefix()` (or add an alternative to do so)

[dktapps]

Description

Instead of returning a single target, it should return all the potential matches. This would allow commands to bail out if there's more than one target, preventing most unintended targeting.

Justification

getPlayerByPrefix() is used by almost all commands. This is dangerous, because the result may suddenly change if a new player joins the server, meaning that reusing a previously-typed command may do something unintended.

This is described here: https://github.com/pmmp/PocketMine-MP/blame/b2aa6396c3cc2cafdd815eacc360e1ad89599899/src/Server.php#L606

Alternative methods

That being said, it's still possible for problems to arise in the example described if SteveJ quits the server, as commands referring to SteveJ would then target SteveJobs as the only online target. So maybe it's better to just get rid of this feature entirely in favour of typing out full player names and/or using command selectors (needs #6519).