RFR: 8298318: (fs) APIs for handling filename extensions [v8]

[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: Add parameter checks for withExtension

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16226/files
  - new: https://git.openjdk.org/jdk/pull/16226/files/f66373e5..8abc295f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16226&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16226&range=06-07

  Stats: 23 lines in 2 files changed: 18 ins; 0 del; 5 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