Adopt `Sendable` (enable native Swift 6 support)

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Swift 6 introduced strict concurrency checking to prevent data-races.

Developers adopting and utilizing Swift 6 features will get compiler errors if they use this library because it doesn't explicitly conform to Sendable, which makes it unusable without manual intervention (or using @preconcurrency on imports, which shouldn't be used because this library uses concurrency).

[thoven87]

There's a PR opened for this issue for about 3 months or so.

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That PR (#274) is a step forward, however it doesn't fully support Swift 6 in its current state.