Change AbstractParser.php to support all Json encoded strings

[AZoumis]

According to the RFC, (https://datatracker.ietf.org/doc/html/rfc7644#section-3.4.2.2) the comparison value rules are built on JSON Data Interchange format ABNF rules. That means that a comparison value such as "test\"" should be parsed as test". However, the current implementation would not recognize this comparison value as a string.

Therefore, the following changed were made:

1. Use a more complex regex that should recognize all JSON encoded strings (https://stackoverflow.com/questions/32155133/regex-to-match-a-json-string).
2. Use json_decode() instead of trim() to decode a string comparison value. This fixes several issues, such as escaped quotes being trimmed.