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 25 minutes and 28 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Build
passed #61681
linux kf5-qt5 qt5.12

00:25:28

35.0%