[Suggestion] Allow `RequiredArgumentBuilder` to be extended

[willkroboth]

This PR makes RequiredArgumentBuilder's constructor protected, which makes it possible to create a class that extends RequiredArgumentBuilder.

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Currently, while RequiredArgumentBuilder is a public, non-final class, its only constructor is private:

https://github.com/Mojang/brigadier/blob/b92c420b2a292dd5c20f6adfafff5e21b9835c6d/src/main/java/com/mojang/brigadier/builder/RequiredArgumentBuilder.java#L11-L25

This makes it impossible to extend RequiredArgumentBuilder, since there is no valid accessible signature for the super call in a subclass.

On the contrary, LiteralArgumentBuilder's constructor is protected:

https://github.com/Mojang/brigadier/blob/b92c420b2a292dd5c20f6adfafff5e21b9835c6d/src/main/java/com/mojang/brigadier/builder/LiteralArgumentBuilder.java#L9-L21

Subclasses are allowed to access protected members of their superclass, so it is possible to extend LiteralArgumentBuilder.

Since RequiredArgumentBuilder and LiteralArgumentBuilder are not final, it seems to me that were intended to be extendable.

[willkroboth]

I realized this is exactly the same PR as #122. I don't think I'll close this though, since double the PRs means double the chance this gets noticed by Mojang :P.