Add valueOrRethrow to Result

[natebosch]

This is the synchronous version of asFuture. Typical uses of Result
are asynchronous and the asFuture API is sufficient, but we already
have the APIs available for checking isValue and reading the value
with .asValue?.value so a synchronous counterpart is easy to
implement.

[github-actions]

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async	Breaking	2.13.1	2.14.0-wip	2.14.0-wip	✔️

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pkgs/async/lib/src/result/error.dart	💚 88 % ⬆️ 1 %
pkgs/async/lib/src/result/result.dart	💚 84 %
pkgs/async/lib/src/result/value.dart	💚 82 % ⬆️ 3 %

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// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
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[natebosch]

We could also write this as an extension getter easily, but I think that these classes were not designed to be extended and it may be worth the potentially breaking change to mark them final. If something comes up in internal usage we may want to rethink this plan, but hopefully no one will have used these as mocks or anything like that.

#970