RFR: 8298318: (fs) APIs for handling filename extensions

[Brian Burkhalter]

Add to `java.nio.file.Path` a method `getExtension` to retrieve the `Path`'s extension, and companion methods `removeExtension` and `addExtension`.

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Commit messages:
 - 8298318: (fs) APIs for handling filename extensions

Changes: https://git.openjdk.org/jdk/pull/16226/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16226&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298318
  Stats: 237 lines in 2 files changed: 236 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16226.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16226/head:pull/16226

PR: https://git.openjdk.org/jdk/pull/16226