RFR: 8355720: Implement JEP 506: Scoped Values

[Andrew Haley]

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

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Commit messages:
 - 8355720: Implement JEP 506: Scoped Values
 - Implement JEP 506: Scoped Values

Changes: https://git.openjdk.org/jdk/pull/24923/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24923&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355720
  Stats: 11 lines in 2 files changed: 1 ins; 5 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/24923.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24923/head:pull/24923

PR: https://git.openjdk.org/jdk/pull/24923