[Enhancement] Add syntax highlighting for Markdown comment blocks

[JEP 467](https://openjdk.org/jeps/467) has a section for *Syntax highlighting and embedded languages*. [TM4E](https://github.com/eclipse-tm4e/tm4e) does syntax highlighting for languages, perhaps it's possible to use it. Or, for Java, there's already a syntax highlighter built in from JDT, so maybe it can be used.

Start by reading JEP 467's section on syntax highlighting and embedded languages, then review TM4E's capabilities and the syntax highlighter already built into JDT. Determine which approach fits Markdown comment blocks in the Java editor and define the expected highlighted result before implementation. Done means the chosen approach highlights supported content reliably in those blocks.

Written by the indexing model from the issue text.