<i18n dev> RFR: 8364365: HKSCS encoder does not properly set the replacement character

[Volkan Yazici]

Fix `HKSCS` encoder to correctly set the replacement character, and add tests to verify the `CodingErrorAction.REPLACE` behavior of all available encoders.

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

Commit messages:
 - Add test to verify `CodingErrorAction.REPLACE` for all charsets
 - Fix incorrect `HKSCS::repl` override

Changes: https://git.openjdk.org/jdk/pull/26635/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26635&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364365
  Stats: 483 lines in 3 files changed: 472 ins; 9 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26635.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26635/head:pull/26635

PR: https://git.openjdk.org/jdk/pull/26635