RFR: 8322708: Global HTML attributes are not allowed

[Nizar Benalla]

Can I please get a review for this change, that aims to add support for Global HTML tags.
Here is the [link](https://cr.openjdk.org/~nbenalla/javadocGlobalAttrs/pkg1/package-summary.html) to the generated docs.
Thanks in advance.

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Commit messages:
 - Allow global variables

Changes: https://git.openjdk.org/jdk/pull/19652/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19652&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322708
  Stats: 613 lines in 5 files changed: 610 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/19652.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19652/head:pull/19652

PR: https://git.openjdk.org/jdk/pull/19652