Added validation for '0x'-prefixed hex strings.

[ayala-io]

• Modified isHexdecimal to allow validation of hexdecimal strings prefixed with the standard '0x'. The '0x' prefix is not required but if it does exist in the string, it is still OK.
• Updated validator.js to include a few more test cases involving the '0x' prefix

Update:

• Added '0h' prefix as a valid hexadecimal format.

[profnandaa]

LGTM. Thanks for the contrib! 🎉

[rlaace423]

This breaks previous work flow because there's someone make use of previous regex (without '0x, 0h' checking). For me, I've used my another function 'isStrictHexadecimal' which checks both 0x prefix and 2-divisible.

If there's migration note for v11 to v12, this changes should be in that.

to make it clearer..
[v11]
val.isHexadecimal('0x1234'); ==> FALSE
val.isHexadecimal('1234'); ==> TRUE

[v12]
val.isHexadecimal('0x1234') ==> TRUE
val.isHexadicimal('1234') ==> TRUE