RichTextComposerNg: port QRegExp to QRegularExpression

If a capture group isn't going to be used, make it clustering/
non-capturing (?:).

Relevant unit tests pass.
1 job for !4 with work/ahmad/qre in 49 minutes and 9 seconds (queued for 26 minutes and 51 seconds)
detached
Status Job ID Name Coverage
  Build
passed #62720
linux kf5-qt5 qt5.12

00:49:09

35.0%