"Use this package" to reference swift package commands

[MahdiBM]

We have a new add-dependency command in Swift 6:

swift package add-dependency https://github.com/DiscordBM/DiscordBM --from 1.12.0

SPI's "Use this package" view should include this, with a small note that it requires Swift 6 CLI.

crates.io also references these commands, which reminded me that we recently got them in Swift too, so SPI can also do that:

[daveverwer]

Thanks for this @MahdiBM, I knew about the new command but hadn't thought properly about how best to integrate it here. I'll promote this to an issue so we make sure that happens.

[daveverwer]

Tracked in #3189