RFR: 8331947: Preview creates checkbox for JEP-less preview feature [v2]

[Hannes Wallnöfer]

> Please review a simple patch to exclude preview visitor classes meant to support future preview features from the Preview API page. 
> 
> The test adds an sample element annotated with the new `PreviewFeature.Feature.LANGUAGE_MODEL` constant (which does not have a `@JEP` annotation) to make sure it is not listed in the Preview API page. The test itself does not have to be modified, as it would fail without the change in `PreviewAPIListBuilder.java`.

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

  Only include preview features with a JEP

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19344/files
  - new: https://git.openjdk.org/jdk/pull/19344/files/5e7cd0de..c3a92a2f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19344&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19344&range=00-01

  Stats: 89 lines in 7 files changed: 34 ins; 13 del; 42 mod
  Patch: https://git.openjdk.org/jdk/pull/19344.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19344/head:pull/19344

PR: https://git.openjdk.org/jdk/pull/19344