RFR: JDK-8298405: Support Markdown in Documentation Comments [v43]

[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:

  Refactor most of TestMarkdown.java into separate tests, grouped by functionality

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16388/files
  - new: https://git.openjdk.org/jdk/pull/16388/files/d460ee33..398f93fc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=42
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=41-42

  Stats: 2001 lines in 9 files changed: 1143 ins; 857 del; 1 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