RFR: 8348282: Add option for syntax highlighting in javadoc snippets

[Hannes Wallnöfer]

Please review a change to add a `javadoc` option to enable syntax highlighting for code fragments in snippets and  `<pre><code>` tags. The new `--syntax-highlight` option uses the [highlight.js] JavaScript library and supports Java, Properties, XML, HTML and JSON. The option is enabled in the JDK API docs target, [generated output is available here](https://cr.openjdk.org/~hannesw/8348282/api.00/index.html).

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

Commit messages:
 - Remove trailing spaces
 - Brigthen variable color
 - Fix regular expression for type arguments
 - 8348282: Add option for syntax highlighting in javadoc snippets

Changes: https://git.openjdk.org/jdk/pull/24417/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24417&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348282
  Stats: 3531 lines in 13 files changed: 3516 ins; 3 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/24417.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24417/head:pull/24417

PR: https://git.openjdk.org/jdk/pull/24417