RFR: 8355720: Implement JEP 506: Scoped Values

[Chen Liang]

On Mon, 28 Apr 2025 14:32:04 GMT, Andrew Haley <aph at openjdk.org> wrote:

> Propose to finalize scoped values.
> The only functional change is that the orElse() method no longer accepts a null argument.

Nit: is the no-arg constructor of `Snapshot` supposed to be `private` instead?

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PR Comment: https://git.openjdk.org/jdk/pull/24923#issuecomment-2835642166