separate dynamic StringDetect rule to avoid copies in StringDetect::doMath()

The ternary in the previous code makes a systematic call to the constructor of QString:

const auto &pattern = m_dynamic ? replaceCaptures(....) : m_string;
                                      ^ QString            ^ QString &

Common type is QString, m_string is implicitly transformed to QString(m_string) (pattern is a reference to a temporary object with an extended lifetime)

The creation of DynamicStringDetect allows a small additional gain.

highlighter_benchmark:

_	Time
before	~22.7s
after	~22.2s (-2%)