add links property to config file

[samdeane]

Instead of having to add link and unlink commands to the .xpkg config file, it should be possible to just list the links with a links property.

This would allow us to replace

{
    "install": [
        ["link", "startup.sh", "~/.local/share/bash-hooks/startup/swift"],
        ["link", "completion.bash", "~/.local/share/bash-hooks/interactive/swift-completion"]
    ],
    "remove": [
        ["unlink", "startup.sh", "~/.local/share/bash-hooks/startup/swift"],
        ["unlink", "completion.bash", "~/.local/share/bash-hooks/interactive/swift-completion"]
    ]
}

with the more compact:

{
    “links”: [
        ["startup.sh", "~/.local/share/bash-hooks/startup/swift"],
        ["completion.bash", "~/.local/share/bash-hooks/interactive/swift-completion"]
    ]
}```