Operator madness & QoL

* Adds an `in` operator, testing if the lefthand side exists in the righthand side. Evaluates to `true`/`false`.
* Overloads the addition operator to allow for more intuitive array concatenation/appending.
* Allow readonly indexing of strings as a shorthand form of `String.prototype.charAt()`
* Fixed typo in BinaryOperatorExpression.cs
* Allow underscores as digit separators in number literals.