WIP: General improvements to syntax highlighter module

• Use modern QRegularExpression objects instead of QRegExp [1]
• Use C++ raw string literals for representing regular expressions
• Promoted even_quotes_pattern regular expression to class member
• Use single regular expression rule for Lua keywords matching
• Simplified all other regular expressions (strings and comments)
• Re-order rules application: keywords -> single-line comments -> strings
• Modernise highlightBlock() function using recent examples [2] [3]
• Use even_quotes_pattern for skipping multi-line comments in strings

Tested this myself. Any feedback from veteran Quest Editor users/developers is welcome.

The syntax highlighter is still not 100% perfect but I think is better than before.

Fixes #472

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Before:

After: