RFR: JDK-8298405: Support Markdown in Documentation Comments [v15]

[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 incrementally with four additional commits since the last revision:

 - MarkdownTransformer: tweak doc comments
 - MarkdownTransformer: change `Lower.replaceIter` to be `private final`
 - MarkdownTransformer: use suggested text for using streams
 - remove obsolete debug code

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16388/files
  - new: https://git.openjdk.org/jdk/pull/16388/files/586daddf..54d40b20

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=13-14

  Stats: 26 lines in 2 files changed: 2 ins; 13 del; 11 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