RFR: 8322708: Global HTML attributes are not allowed [v3]

[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/javadocGlobalPR/pkg1/package-summary.html) to the generated docs.
> Thanks in advance.

Nizar Benalla 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 ten additional commits since the last revision:

 - Null safety
 - Merge branch 'master' into html-attributes
 - remove trailing whitespace
 - -Add a boolean attribute to the enum type
   -Simply regex in `visitAttribute`
   -Simplified the Test
   -Added a negative test
 - no longer print summary
 - no longer print summary
 - Add small comment
 - Remove classpath exception
 - Allow global variables

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/19652/files
  - new: https://git.openjdk.org/jdk/pull/19652/files/a1292496..248f1f2a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19652&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19652&range=01-02

  Stats: 24208 lines in 691 files changed: 13960 ins; 8261 del; 1987 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