Optimize several regular expressions in RegexHelper.php for better performance:

• REGEX_PUNCTUATION: Simplified from explicit character list to Unicode character classes [\p{P}\p{S}], removing redundancy
• REGEX_THEMATIC_BREAK: Consolidated anchors and restructured alternation for more efficient matching
• PARTIAL_ENTITY: Added atomic group (?>#...) to prevent catastrophic backtracking during entity parsing
• HTML patterns: Applied possessive quantifiers (*+, ++) to PARTIAL_OPENTAG, PARTIAL_CLOSETAG, PARTIAL_OPENBLOCKTAG, PARTIAL_CLOSEBLOCKTAG, and PARTIAL_LINK_TITLE to eliminate unnecessary backtracking
• HTML block regexes: Updated TYPE_6_BLOCK_ELEMENT with possessive quantifiers for better performance on malformed HTML

Fixes #674

(Note that we don't consider changes to regex constants to be BC breaks, so long as they continue to correctly parse what they're supposed to)