Allow to install a list of versions

[hasufell]

No description provided.

[dfordivam]

Just to clarify this means the following should install both versions right?
And it should fail if ghcup cannot install any one of them.

ghcup install ghc 9.6.6 9.8.4

[dfordivam]

As the install command also supports CONFIGURE_ARGS, without any use of --, the versions cannot be specified as space separated.
This is difficult to parse as two separate versions, followed by config args.
ghcup install ghc 9.8.10 9.8.8 -- --config-args
Currently this will pass "9.8.8" and "--config-args" both to config.

Perhaps comma separated list would work
ghcup install ghc 9.8.10,9.8.8 -- --config-args

[dfordivam]

The installation of multiple versions will typically need to happen only for ghc, and -u, -i and --set will be incompatible/error if multiple versions are specified.

[hasufell]

The installation of multiple versions will typically need to happen only for ghc, and -u, -i and --set will be incompatible/error if multiple versions are specified.

Why would -i be incompatible? --set is also fine... the last version will "win".

-u may need smarter parsing, but I think it should be possible with optparse.

[hasufell]

As the install command also supports CONFIGURE_ARGS, without any use of --, the versions cannot be specified as space separated.

I think there's no way to only allow something as longopts only, is there?