Editora 0.18.3. Archived, so links from that release keep working. Read the 0.18.5 page →
Edit: Untabify Region
edit.untabify Expand tabs to spaces over the selection (or the whole buffer).
How to run it
Open the command palette (M-x in the Emacs keymap) and search for Edit: Untabify Region, then press Enter. It has no default keybinding, but you can bind one in Settings → Keymaps.