RFR: 8298318: (fs) APIs for handling filename extensions [v9]

[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 12 additional commits since the last revision:

 - 8298318: Update @since tags
 - Merge
 - 8298318: Add parameter checks for withExtension
 - 8298318: Revise handling of leading periods
 - 8298318: corretions to commit 030a250aaf4c309d515507527932b233f210501e
 - 8298318: Address reviewer comments
 - 8298318: Remove vestigial trim() call and commented out methods
 - 8298318: Refine specification of extension; replace addExtension/removeExtension with withExtension/withoutExtension; update test
 - Merge
 - 8298318: Correct type in path.getExtension spec
 - ... and 2 more: https://git.openjdk.org/jdk/compare/348fa9b5...b27cf346

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16226/files
  - new: https://git.openjdk.org/jdk/pull/16226/files/8abc295f..b27cf346

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16226&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16226&range=07-08

  Stats: 693830 lines in 8322 files changed: 174385 ins; 165771 del; 353674 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