feat: make the Theme type support sendablity

[MojtabaHs]

The type must be sendable or it will be an error in Swift 6

[MojtabaHs]

A simple sample code for test in Swift 6:

extension Theme {
    static let myTheme = Theme()
}

[gonzalezreal]

Hi,
Thanks for looking into this!

I think there is more to it than making Theme conform to Sendable. When you create a Package@swift-6.0.swift with // swift-tools-version:6.0 you will see a few more issues.

I already went through this process for gonzalezreal/NetworkImage. See gonzalezreal/NetworkImage#34.

I didn't have time to do the same with MarkdownUI. You are welcome to give it a try. There is an attempt in #349.

[MojtabaHs]

I reviewed that PR already, but it seems too-much-changed to review and merge all at once. I needed this functionality in my project, and it worked with just this one simple change. There’s no need to mark everything as Sendable or raise the minimum Swift version in my case.

In my opinion, we should migrate step by step.
Let me know your opinion. thanks

[gonzalezreal]

Sure! I agree. I was misled by the PR title. My understanding is that this PR is about making Theme conform to Sendable, not compatible with Swift 6. Making it compatible with Swift 6 can require more work.

[MojtabaHs]

Absolutely! With this change, you'll be able to use the Theme type in code with stricter concurrency checks, including Swift6-only codebases. However, not all features of the framework will be fully compatible yet. More work is needed for full compatibility, just as you mentioned.

[MojtabaHs]

Thanks for merging, please don't forget to tag and release this (maybe 2.4.2) so that I can continue using this fantastic library directly.

Thanks again 🙌🏻