RFR: JDK-8298405: Implement JEP 467: Markdown Documentation Comments [v49]

[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 68 commits:

 - Merge with upstream/master
 - improve first-sentence break in Markdown comments
 - add test cases for links to anchors
 - fix `textOf` method to accommodate Markdown content.
 - avoid relying on unspecified behavior `List.toString`
 - 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
 - ... and 58 more: https://git.openjdk.org/jdk/compare/586396cb...76eccbf4

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Changes: https://git.openjdk.org/jdk/pull/16388/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=48
  Stats: 23757 lines in 208 files changed: 23030 ins; 281 del; 446 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