RFR: 8298318: (fs) APIs for handling filename extensions [v5]

[Brian Burkhalter]

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

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  8298318: Address reviewer comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16226/files
  - new: https://git.openjdk.org/jdk/pull/16226/files/caeb1f14..030a250a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16226&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16226&range=03-04

  Stats: 66 lines in 2 files changed: 37 ins; 16 del; 13 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