RegExp ES6 implementation incomplete

ES6 regular expression prototype and symbol features were never fully implemented or enabled - this is the one signficant ES6 gap that Chakracore still has.

The two flags: -ES6RegexPrototypeProperties and -ES6RegexSymbols guard much of this incomplete work.

These need a thorough review/clean up and then to be enabled, ideally for the next release.

This will solve the following issues which I have closed to consolidate here:
#6342
#5679
#5629
#5579
#5512
#5510
#5508
#5388
#5187
#6364