RFR: 8298318: (fs) APIs for handling filename extensions [v3]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - 8298318: Refine specification of extension; replace addExtension/removeExtension with withExtension/withoutExtension; update test
 - Merge
 - 8298318: Correct type in path.getExtension spec
 - Merge
 - 8298318: (fs) APIs for handling filename extensions

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16226/files
  - new: https://git.openjdk.org/jdk/pull/16226/files/5a926bdc..b7f00bb3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16226&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16226&range=01-02

  Stats: 288681 lines in 6308 files changed: 155902 ins; 95805 del; 36974 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