Update parser to allow for inline conditional returns

[alexdovzhanyn]

Currently if you try to write a conditional such as:

if (someCondition) return 'hello'

the compiler will throw a ParseError because it expects to find a block or an expression after the condition, but return is currently not being treated as an "expression". Let's change the parsing for conditional nodes, to allow for returns.

• Update the parser to make returns work as the expression (for if, else if, and else)
• Write a test to validate that the return keyword works in a single-line conditional with no curly braces