I can't imagine a sandbox would help. what can an an extension do that doesn't touch some arbitrary code that gets run? it could add a line to the middle of a giant file right before you run and remove it immediately after. even if you run the whole editor in a sandbox you do eventually deploy that code somewhere, it can change something inconspicuous like a url in a dependency file that might not get caught in a pr
the only solution is to audit everything you install, know all the code you run, etc. ofc that's not reasonable, but idk what else there is. better automated virus check things maybe? identity verification for extension publishers? idk if there's an actual solution
vaultwarden is already reasonably mature for server