feat(package:async): migrate the Result class to sealed class

[utamori]

By changing the result class from an abstract class to a sealed class,
it will be possible for users to perform exhaustive checks.

#364

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[lrhn]

It's not a bad idea, but it is also potentially breaking.

I'd do it as part of a larger change that adds class modifiers everywhere where they belong in the package, which may require a major version increment.
(Would probably also make the ValueResult and ErrorResult classes final at that point.)