RFR: 8325168: JShell should support Markdown comments

[Jan Lahoda]

This is an attempt to add Markdown support in documentation comments to JShell.

It works by converting the Markdown text to HTML during the process of resolving `{@inheritDoc}` tags.

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Depends on: https://git.openjdk.org/jdk/pull/16388

Commit messages:
 - Cleanup.
 - Preventing a NPE.
 - Fixing test.
 - Merge branch '8298405.doclet-markdown-v3' into JDK-8298405
 - Cleanup.
 - Merge branch '8298405.doclet-markdown-v3' into JDK-8298405
 - Cleanup.
 - Merge branch '8298405.doclet-markdown-v3' into JDK-8298405
 - Removing ToHTMLMarkdownTransformer, as suggested
 - Adding forgotten class.
 - ... and 3 more: https://git.openjdk.org/jdk/compare/a8ea1c72...0ba0fffd

Changes: https://git.openjdk.org/jdk/pull/17686/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17686&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325168
  Stats: 1825 lines in 8 files changed: 977 ins; 842 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/17686.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17686/head:pull/17686

PR: https://git.openjdk.org/jdk/pull/17686