Create ghcup meta information file

Purpose of meta information file:

• detect new GHC and cabal-install versions on-the-fly without requiring users to update the script in-place
• download reliability: meta information may contain download url information in addition to versions

Questions to answer:

1. where should this file be hosted? (downloads.haskell.org or this repo?)
2. which format? (should be easily parsable/sourcable by shell/awk or do we want to go for json/toml and use dependencies for parsing)
   • consider that the format might be extended in the future
3. what information should it hold exactly?
   • available ghc/cabal-install versions?
   • latest ghc/cabal-install version?
   • "good" or "bad" versions (e.g. known buggy releases) and reason
4. what is the method/process for updating this meta information file?