RFR: JDK-8298405: Support Markdown in Documentation Comments [v45]

[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 63 commits:

 - fix test after merge
 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3
 - Revise `Markdown.update` to better handle container blocks.
 - Refactor most of TestMarkdown.java into separate tests, grouped by functionality
 - add support for (top-level) trailing code blocks
   add simple test case with tabs
   add TestMarkdownCodeBlocks.testTypical
 - fix whitespace
 - fix Markdown.update for new POST_LIST_INDENT test case given in review feedback
 - refactor tests for Markdown headings into a separate test class.
 - update DocCommentParser and tests to improve handling of code blocks and code spans in Markdown documentation comments
 - fix indentation, for consistency
 - ... and 53 more: https://git.openjdk.org/jdk/compare/6f8d351e...292ff0f1

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Changes: https://git.openjdk.org/jdk/pull/16388/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=44
  Stats: 23548 lines in 205 files changed: 22863 ins; 252 del; 433 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