Regex: implementation of the \x \X BSLS, issue #21607

[penguindark]

What's inside
Implementation on the \x and \X BSLS

• regex added \xAF and \X1234 BLSL match
• \x12 match a byte with the hex value 0x12
• \X1234 match two consecutive bytes with the hex values 0x12 0x34
• more tests
• some code cleaning
• solve the issue regex.regex_opt doesn't handle \x escape codes #21607

[spytheman]

Excellent work.