RFR: JDK-8298405: Support Markdown in Documentation Comments [v18]

[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 one additional commit since the last revision:

  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.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16388/files
  - new: https://git.openjdk.org/jdk/pull/16388/files/f086aaab..203532b2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=17
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=16-17

  Stats: 191 lines in 11 files changed: 89 ins; 90 del; 12 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