RFR: 8335991: Implement Simple Source Files and Instance Main Methods (Fourth Preview)

[Jan Lahoda]

This is a partial implementation of JEP 495 - adjustments of the JEP metadata in `PreviewFeature`. There are no language changes associated with this JEP. Changes to the `java.io.IO` class are covered by https://github.com/openjdk/jdk/pull/21693.

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Commit messages:
 - 8335991: Implement Simple Source Files and Instance Main Methods (Fourth Preview)

Changes: https://git.openjdk.org/jdk/pull/21787/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21787&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335991
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21787.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21787/head:pull/21787

PR: https://git.openjdk.org/jdk/pull/21787