feature request: support multiple ghc versions

It would be nice to support multiple ghc versions; a common pattern is to e.g. have multiple `stack.yaml`s and each is associated to a ghc version; so perhaps we could have something like; 
`ghcVersions = [ [8 10 7] [9 0 2] [9 2 4] [9 4 4] ]` `toStackLockFileName = (x: "stack-${majorVersionString x}.lock.yaml")` and then have it automatically discover all stack locks in the repo. 