implement exponentiation operator

[arbroween]

The javascript built-in exponentiation operator (**) is missing in the implementation of the lexer. It is a shorthand for Math.pow() built-in.

It should probably be added to the lexer in this method, somewhere in the big match statement https://github.com/jasonwilliams/boa/blob/master/src/lib/syntax/lexer.rs#L207-L533

The operator's syntax and semantics is defined in this section here https://tc39.es/ecma262/#sec-exp-operator in the spec and does the same thing at runtime than Math.pow(), which is already implemented here https://github.com/jasonwilliams/boa/blob/master/src/lib/js/math.rs#L140-L148.

[arbroween]

This issue could maybe also be labelled as good first issue

[cpyle0819]

Re: 'somewhere in the big match statement'. There's already a * operator, so ** should probably be nested within that similar to \\.

The * match is here: https://github.com/jasonwilliams/boa/blob/master/src/lib/syntax/lexer.rs#L479

[cpyle0819]

Now that I look at it, this looks done.

[oll3]

The ** should also be added as an assign op (**=)

[simonbrahan]

Mind if I take a look at this one?