RFR: 8340404: Clarify CharsetProvider deployment on application module path

[Justin Lu]

Please review this PR and [CSR](https://bugs.openjdk.org/browse/JDK-8340412) which clarifies that a CharsetProvider SPI can be provided via the application module path.

This came as a result from a comment on the PR of JDK-8339735. 

I included some minor wording on deploying as a module, with a link to `Service Loader's` _Developing Service Providers section_ to handle the heavy lifting. However, I am happy to adjust as needed, if it is felt that more info should be laid out directly on `CharsetProvider's` class description itself.

An associated test is provided to confirm that a `CharsetProvider` _can_ be deployed as a module.

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Commit messages:
 - newline in module-info.java
 - init

Changes: https://git.openjdk.org/jdk/pull/21076/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21076&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340404
  Stats: 149 lines in 4 files changed: 139 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/21076.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21076/head:pull/21076

PR: https://git.openjdk.org/jdk/pull/21076