Ability to specify custom Vercel environment

Same issue as here: https://github.com/BetaHuhn/deploy-to-vercel-action/issues/396

Vercel now supports custom environments beyond Dev, Preview and Production, see https://vercel.com/docs/cli/deploy#target

There's a PR (https://github.com/BetaHuhn/deploy-to-vercel-action/pull/397) proposed in the originally forked repo, would you consider merging it here too?