Escape Meta-characters like $, ^ in Regular Expression provided in pattern statement

Pattern statement (RFC 6020, Section 9.4.6) accepts regular expression defined by XSD Schema Spec (https://www.w3.org/TR/2004/REC-xmlschema-2-20041028/#regexs) which doesn't have meta-characters like ^ and $.

XSD Schema definition of RegExp expects to match the entire string and so Yangtools is correctly returning the pattern defined in Yang enclosed within ^ and $. But, it doesn't escape these characters if they are found inside the pattern value.

Due to this issue, attempts to use PatternConstraint::getRegularExpression as is with Java Pattern/Matcher classes doesn't work as expected.