Add Swift Package Manager support

[woxtu]

Hi,

This PR adds Swift Package Manager support to this library.

Close #23

[victormihaita]

@anatoliyv @woxtu Is this something that can be merged?

[victormihaita]

please merge this PR when you can. Is one of the few dependencies missing from my project

[acosmicflamingo]

Do you guys hear that? It's a crowd chanting!

"Let's Go AssistantKit Developers; Let's Go!" thump thump thump thump thump

✺◟( ͡° ͜ʖ ͡°)◞✺ ✺◟( ͡ ͜ ʖ ͡ )◞✺ ✺◟(◕‿◕✿)◞✺ ✺◟( ͡ ͜ ʖ ͡ )◞✺ ✺◟( ͡° ͜ʖ ͡°)◞✺ ✺◟( ͡ ͜ ʖ ͡ )◞✺ ✺◟(◕‿◕✿)◞✺ ✺◟( ͡ ͜ ʖ ͡ )◞✺ ✺◟( ͡° ͜ʖ ͡°)◞✺

We got this!!! Let's bring it home! Let's merge this PR :D