<i18n dev> RFR: 8364365: HKSCS encoder does not properly set the replacement character [v2]

[Volkan Yazici]

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

Volkan Yazici has updated the pull request incrementally with two additional commits since the last revision:

 - Improve double-`char` unmappable search
 - Improve single-`char` unmappable search

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26635/files
  - new: https://git.openjdk.org/jdk/pull/26635/files/4956a5b1..f567f2c8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26635&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26635&range=00-01

  Stats: 55 lines in 2 files changed: 3 ins; 37 del; 15 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