RFR: 8348282: Add option for syntax highlighting in javadoc snippets [v3]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'master' into JDK-8348282
 - Adjust CSS spacings, remove whitespace
 - Remove trailing spaces
 - Brigthen variable color
 - Fix regular expression for type arguments
 - 8348282: Add option for syntax highlighting in javadoc snippets

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24417/files
  - new: https://git.openjdk.org/jdk/pull/24417/files/2700131d..af50d173

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24417&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24417&range=01-02

  Stats: 21918 lines in 734 files changed: 15501 ins; 4680 del; 1737 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