JetBrains: Add check to plugin publish process that we're on `main`

Created by: vdavid

Add an additional check to scripts/release.sh to make sure that we’re making the release from main.

The intention is to change the process of changes → version bump → release → peer review → merge to changes → version bump → peer review → merge → release. This slows down the process a bit but gives an additional layer of security that only peer-reviewed code is released.

Note: without checking that we have no commits on main that don’t exist on origin/main, this is not a foolproof check, and the process could be abused. We’re supposing that all our devs have good intentions, plus that they won’t locally commit directly to main by accident without noticing it before making a release.