Make package safe for Swift 6

[ptoffy]

So after #48 and the introduction of Swift 6, some @unchecked Sendable annotations were added. While this compiles, it's not the correct way to approach the safety concerns introduced by Swift 6. This PR attempts to fix that by (over?)annotating the package with Sendable. I think this is a thing that should be done before releasing the latest major version as we're free to make breaking changes.
The PR is currently in a semi-finished state as it should work as it is, but it's difficult to tell without tests. I also think that some improvements can be made to this solution as we do still have a few classes and mutable state around but I'm open to discussions about it

Also adds a build workflow for main and PRs

[ptoffy]

@MihaelIsaev there's a batchSend method in the README I can't find in the repo. I don't need it but if it's been removed it might make sense to remove it from the README too

[MihaelIsaev]

@ptoffy Yeah it's been removed in #48 according to #45 so yeah it should be removed from the README as well

[MihaelIsaev]

@ptoffy Have you tried it in action with Swift 6? I don’t have a chance to test it right now, but if everything works well, I’m ready to merge and tag it as v3.

[MihaelIsaev]

@ptoffy Have you tried it in action with Swift 6?

[ptoffy]

Sorry @MihaelIsaev missed this. I have barely tried it locally and yet have to try it in a staging/prod environment, it will likely be this week. I'd suggest tagging a beta for now

[MihaelIsaev]

@ptoffy Thank you very much for your contribution!
It’s now available in the 3.0.0-beta.1 tag.
Can’t wait for your feedback from tests in staging/prod environments ⚡️