fix symmetricity of CustomEmojiImpl/RichCustomEmojiImpl#equals

[danthe1st]

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Changes

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Closes Issue: NaN

Description

The contract of Object#equals specifies:

It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.

This should hold when calling equals on an instance of RichCustomEmojiImpl and CustomEmojiImpl.

This is because CustomEmojiImpl allows any CustomEmoji and checks the id while RichCustomEmojiImpl expects a RichCustomEmojiImpl.

CustomEmoji a = new RichCustomEmojiImpl(123, someGuild);//using internal classes for demonstration
CustomEmoji b = new CustomEmojiImpl("a",123,true);
assert a.equals(b) == b.equals(a);//false

This PR changes RichCustomEmojiImpl to allow any CustomEmoji instances and CustomEmoji in order to check the emoji name just like RichCustomEmojiImpl.
This allows JDA users to easily compare two CustomEmoji instances.