Add Expression.isResult(child?: Expression)

[ClickerMonkey]

We need a way to know whether an expression is used as a value (commonly referred to as rvalue).

When we're building runtimes that output certain languages as text, things like for/while/do/if/switch can be inline UNLESS it's being used as an rvalue.

The function will look at the parent expression, and run parent.isResult(this). If the child is used as an rvalue in the parent expression return true, otherwise default to the grandparent if one exists.