Concurrency warnings when using swift-markdown with Swift 5.10 strict concurrency enabled

[dempseyatgithub]

Using swift-markdown as a dependency causes concurrency warnings when strict concurrency is enabled in Swift 5.10.

The warnings are primarily because of option set types not being marked as Sendable.

A typical warning:

Static property 'disableSmartOpts' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in Swift 6

Adding Sendable to ParseOptions, Aside.Kind, and MarkupDumpOptions addresses the warnings when these types are used in client projects.