RFR: JDK-8298405: Support Markdown in Documentation Comments [v12]

[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 15 commits:

 - refine the Comment used for synthetic doc comments
 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3
 - address review feedback
 - fix @since tags
 - fix copyright year
 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3
 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3
 - Merge with upstream/master
 - Merge with upstream/master
 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3
 - ... and 5 more: https://git.openjdk.org/jdk/compare/2cd1ba6a...0da7ba5c

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Changes: https://git.openjdk.org/jdk/pull/16388/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=11
  Stats: 21376 lines in 191 files changed: 20712 ins; 271 del; 393 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