RFR: 8298478: (fs) Path.of should allow input to include long path prefix

[Brian Burkhalter]

Remove the long path `\?` or `\?\UNC` prefix before creating the `WindowsPath` instance..

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Commit messages:
 - 8298478: (fs) Path.of should allow input to include long path prefix

Changes: https://git.openjdk.org/jdk/pull/12423/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12423&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298478
  Stats: 23 lines in 2 files changed: 20 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12423.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12423/head:pull/12423

PR: https://git.openjdk.org/jdk/pull/12423