Feature request: install alpha, beta, rc versions of ghc

ghcup install 8.8.1-alpha1

Current behavior: "8.8.1-alpha1" is not a known version or tag!

Desired behavior: Installs the specified ghc alpha, symlinks ghc-8.8.1-alpha1 in ~/.ghcup/bin to the appropriate binary, and does the same for ghc-pkg, etc.

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