RFR: 8338133: Cleanup direct use of `new HtmlTree`

[Jonathan Gibbons]

Please review an update to "clean up" the direct use of HtmlTree constructors.

Hitherto, many/most instances of `HtmlTree` were created by static factory methods.  This update extends that convention.
In most cases, this is by providing either simple no-arg factory methods or commonly used overloads that take an `HtmlId` or `HtmlStyle`.

For some tags, (`br`, `hr`, `wbr`) this allows a singleton instance to be used.
For some of the more obscure cases, a more generic `HtmlTree.of(HtmlTag)` method was used.

Notes:
* some significant block-level nodes, like `pre`, should probably always set a style, which could be enforced by suitable factory methods. That is currently not the case and could be a future cleanup.
* some lists put the same style info on each list item, but might be better placed on the enclosing list. That could be a future cleanup

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Commit messages:
 - JDK-8338133: Cleanup direct use of `new HtmlTree`

Changes: https://git.openjdk.org/jdk/pull/20778/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20778&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338133
  Stats: 310 lines in 28 files changed: 195 ins; 24 del; 91 mod
  Patch: https://git.openjdk.org/jdk/pull/20778.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20778/head:pull/20778

PR: https://git.openjdk.org/jdk/pull/20778