Introduce pager mode (for ghcup list)

[hasufell]

Following #1115 it appears the list is now big enough that it can be annoying to handle. Although users can just issue ghcup list | less, other tools such as git by default open a pager, unless the output is part of a pipe.

So I'm proposing the following behavior:

• new config option pager: { list: bool, cmd: string }
  • cmd is optional and can point to a full path or a filename (to be searched for in PATH)
  • when pager: nil -> paging disabled
  • when pager: { list: false, ... } -> paging disabled for list
  • pager: true is a shorthand for pager: { list: true } turning all boolean options true (if there are future ones)
  • pager: string is a shorthand for pager: { list: true, cmd: string }, turning all boolean options true (if there are future ones)
  • setting cmd to an empty string will disable paging as well
• new cli option ghcup [-p|--paginate|-P|--no-pager]
  • overwrites config options
• lookup of pager in the given order
  • if cmd for pager config option is set, use it
  • otherwise check $GHCUP_PAGER env var
  • otherwise check $PAGER env var
  • otherwise check the following binaries in order
    • most
    • more
    • less
• when to open the pager
  • default is off
  • if paging is enabled through pager config option
  • or if paging is enabled through cli option
• when to not open pager (overwrites all other conditions)
  • if ghcup is run as part of a pipe (e.g. ghcup list | grep foo or bar=$(ghcup list))
  • or if ghcup list --raw-format is passed
  • or if no valid pager is found
• other remarks
  • warnings or anything else going to stderr is not shown in the pager
  • we may want pager for ghcup --help (compare with git --help vs git -h)
  • setting pager to cat should work too (and just print to stdout)

In the future we may:

1. switch the default for paging to true (need community input)
2. but only invoke a pager when the terminal height is less than the output (compare with less -F)

Useful libraries to check:

• https://hackage.haskell.org/package/terminal-size
• https://hackage.haskell.org/package/pager (I don't think I want to depend on this, but it can be an inspiration)