RFR: 8299902: Support for MarkDown javadoc in JShell [v2]

[Jan Lahoda]

> Adding support for MarkDown javadoc in the JShell

Jan Lahoda has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Cleanup.
 - Post-merge updates.
 - Merge branch '8298405.doclet-markdown' into markdown-in-jshell
 - Update copyright years
 - Rename FFFC variable
   Share Markdown parser and renderer in instance of MarkdownHandler
 - Move CommonMark to new internal module.
   Add legal header to imported CommonMark source files
   Always use Text nodes inside AttributeTree values
   Unwrap <p> from "simple" paragraphs
 - Always use Text nodes inside AttributeTree values
 - Update to CommonMark 0.21.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11936/files
  - new: https://git.openjdk.org/jdk/pull/11936/files/89f51aa6..89f51aa6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11936&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11936&range=00-01

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11936.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11936/head:pull/11936

PR: https://git.openjdk.org/jdk/pull/11936