RFR: 8348282: Add option for syntax highlighting in javadoc snippets [v4]

[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).

Hannes Wallnöfer has updated the pull request incrementally with one additional commit since the last revision:

  Adjust color palette

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24417/files
  - new: https://git.openjdk.org/jdk/pull/24417/files/af50d173..05fc1a4a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24417&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24417&range=02-03

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 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