RFR: 8299902: Support for MarkDown javadoc in JShell

[Jan Lahoda]

Adding support for MarkDown javadoc in the JShell

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

Depends on: https://git.openjdk.org/jdk/pull/11701

Commit messages:
 - Cleanup.
 - Post-merge updates.
 - Merge branch '8298405.doclet-markdown' into markdown-in-jshell
 - Improving handling of new trees.
 - Prototype of MarkDown javadoc support for JShell.

Changes: https://git.openjdk.org/jdk/pull/11936/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11936&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299902
  Stats: 1705 lines in 9 files changed: 947 ins; 752 del; 6 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