Fix concurrency issues for Swift 6

[DePasqualeOrg]

This includes a breaking change related to the preloaded case of the Resource enum in Font.swift, which is necessary in order to make Resource sendable.

[DePasqualeOrg]

I'd like to see why the CI checks failed, but the results aren't accessible to me when I click on the "Details" link.

[DePasqualeOrg]

@JohnSundell, it looks like there haven't been any new commits to this repo in over two years. If you're not able to maintain it, I might continue working on my fork of it, where I could try to add grammars for more programming languages.