I would have assumed Sigil would have had a button/tool to do that, but it appears not.
FWIW 1 - The calibre book editor has a button (Smart Punctuation) to do it. And Diap Dealer is developing an
Even Smarter Punctuation plugin for the calibre book editor. You do not have to use the calibre library manager to use the book editor, it can be used stand alone.
FWIW 2 - I use both the Sigil and the Calibre editors.
BR