[Sage Backend] Port QRegExp to QRegularExpression

To match a backspace char with PCRE/QRegularExpression we have to use
\b inside [].

Use QStringLiteral with QRegularExpression ctor, as it doesn't have a
QLatin1String overload and will convert the to QString anyway.

All unittests pass except for:

• testlua (unrelated, fails on master too)
• testworksheet (hangs, here and on master too)
• testsage, fails to pass here and on master