[glox] - Add support for comma operator

[silverhairs]

The comma operator is used to separate multiple expressions within a single statement. It evaluates the left operand, discards the result, evaluates the right operand, and returns the result of the right operand. It is primarily used in situations where multiple expressions need to be executed in a specific order, such as in a for loop's increment section.

Since the operators precedences for lox are more or less the same as those of C, here is a precedence table with a couple of operators used by lox arranged in descending order of precedence:

Precedence	Operator	Description	Associativity
Highest	?:	Ternary conditional	Right to left
	== !=	Equality operators	Left to right
	! ~ ++ -- + -	Unary operators	Right to left
	+ -	Addition, subtraction	Left to right
	< <= > >=	Relational operators	Left to right
	* / %	Multiplication, division, modulo	Left to right
	,	Comma operator	Left to right