Add isNext(char) and isNext(Predicate<Character>) methods to ImmutableStringReader and StringReader

[ErrorCraft]

I see myself and other people use reader.canRead() && reader.peek() == c (or !reader.canRead() || reader.peek() != c) all the time. This is very tedious to do, so this adds the isAt(char) method to the ImmutableStringReader interface and StringReader class.

[ghost]

All CLA requirements met.

[Pokechu22]

I feel like isAt isn't the best name for this (at makes me think of an index, not a character); maybe nextIs or something like that would be better?

[ErrorCraft]

Ah yeah, maybe something like isNext to be more consistent with the other method names like canRead? (It also reads a bit better imo)

[ErrorCraft]

Changed the method name to isNext. I was also thinking of adding an isNext method that accepts a Predicate<Character> as a parameter to be able to check for more than one character. For example canRead() && Character.isWhitespace(peek()) in the skipWhitespace method would become isNext(Character::isWhitespace).

[Marcono1234]

Would it make sense to implement these as default methods of the interface instead?
(Though assuming no or only a few users implement their own ImmutableStringReader it likely makes no difference)

[Marcono1234]

What about using IntPredicate here? This would avoid boxing and unboxing. But maybe it does not matter much. And it might cause confusion, making users believe this handles code points (including > Character.MAX_VALUE), so this would require renaming the parameter to for example charPredicate.

[peterix]

Closing and reopening to rerun checks.