Make Asset and ImageAsset conform to Sendable

[SergeyPetrachkov]

What's done:

• added Sendable conformance to ImageAsset and Asset types so the generated code is Swift6- and strict-concurrency-checks- ready. Since all types that are used in the equation are Sendable, and since Sendable is available iOS 8+, macOS 10.10+, watchOS 2.0+ (in other words from the very old versions), it should be safe to just add Sendable conformance to the swift5 stencil without introducing breaking changes. This will allow people to use the generated codes without any unwanted warnings from the compiler.

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