RFR: 8340404: Clarify CharsetProvider deployment on application module path [v3]

[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.

Justin Lu has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:

 - resolve conflicts; list deployment as module first
 - merge master, resolve conflicts
 - reflect review comments
 - newline in module-info.java
 - init

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Changes: https://git.openjdk.org/jdk/pull/21076/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21076&range=02
  Stats: 154 lines in 4 files changed: 145 ins; 1 del; 8 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