bug#50766: Regexp paren pairs in query-replace-regexp prompt should be highlighted

[martin rudalics]

> Which means I have to prime these before-change to avoid parsing the
> buffer twice.  Darn it!
> So a fix for both has to hook into post-self-insert and after-change.

I attach a preliminary patch that should work for both.  Please give it
some testing.

martin

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