For longer commit messages it would be great to have a spell checker there. Ideally, VSCode would provide that out of the box but if the direction is to leave spell checking up to extensions (and there are already some great ones out there, like Spell Right by @bartosz-antosik), there could be an API for that.

Possibly related: Offline spell checker for VSCode #20266

UPDATE: since 2020, Electron supports spell checking natively, see below.