RFR: JDK-8298405: Support Markdown in Documentation Comments [v22]

[Jonathan Gibbons]

> Please review a patch to add support for Markdown syntax in documentation comments, as described in the associated JEP.
> 
> Notable features:
> 
> * support for `///` documentation comments in `JavaTokenizer`
> * new module `jdk.internal.md` -- a private copy of the `commonmark-java` library
> * updates to `DocCommentParser` to treat `///` comments as Markdown
> * updates to the standard doclet to render Markdown comments in HTML

Jonathan Gibbons has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 28 commits:

 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3 # Please enter a commit message to explain why this merge is necessary, # especially if it
   merges an updated upstream into a topic branch. # # Lines starting with '#' will be ignored, and an empty message aborts # the commit.
 - add test case contributed by @lahodaj
 - initial fix for source offset problem
 - remove unused imports
 - First pass at remove DocCommentTransformer from the public API.
   
   It is still declared internally, and installed by default, using the service-provider mechanism.
   If the standard impl is not available, an identity transformer is used.
 - updates for "first sentence" issues and tests
 - add info about provenance of `jdk.internal.md` module
 - MarkdownTransformer: tweak doc comments
 - MarkdownTransformer: change `Lower.replaceIter` to be `private final`
 - MarkdownTransformer: use suggested text for using streams
 - ... and 18 more: https://git.openjdk.org/jdk/compare/18e24d06...63dd8bf4

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

Changes: https://git.openjdk.org/jdk/pull/16388/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=21
  Stats: 21713 lines in 193 files changed: 21042 ins; 274 del; 397 mod
  Patch: https://git.openjdk.org/jdk/pull/16388.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16388/head:pull/16388

PR: https://git.openjdk.org/jdk/pull/16388