add remainder (% aka modulus) operator

[cometkim]

See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Remainder
See also https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_precedence

Resolved #6455

[cristianoc]

Looks great.
Just need to double-check possible conflicts of adding %.
E.g.3 % raw("0") parses as binary operator. It seems OK, but worth thinking about a little bit.

[cometkim]

Looks great. Just need to double-check possible conflicts of adding %. E.g.3 % raw("0") parses as binary operator. It seems OK, but worth thinking about a little bit.

Yes, I noticed that %extension had a little ambiguity about space, but we're already enforcing % ext to be formatted to %ext.