RFR: 8298318: (fs) APIs for handling filename extensions [v2]

[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 three additional commits since the last revision:

 - 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/b34c9378..5a926bdc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16226&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16226&range=00-01

  Stats: 685919 lines in 3006 files changed: 119418 ins; 488709 del; 77792 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