RFR: 8266666: Implementation for snippets [v25]

[Pavel Rappo]

> This PR implements JEP 413 "Code Snippets in Java API Documentation", which hasn't been yet proposed to target JDK 18. The PR starts as a squashed merge of the https://github.com/openjdk/jdk-sandbox/tree/jdk.javadoc/snippets branch.

Pavel Rappo has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 37 commits:

 - Merge branch 'master' into 8266666
 - Improve markup error messages
   
    * Cleans up error string format
    * Internationalizes error messages, mapping them to fewer resource keys
    * Fixes a few bugs related to error messages, including: inaccurate caret positioning and runaway resource name (i.e. the resource key wasn't translated into an error)
 - Merge branch 'master' into 8266666
 - Address trivial feedback
 - Address feedback
   
   1. Reverts changes from CommentUtils.java (unused methods)
   2. Changes misguided regex-related assertion to tests
 - Remove trailing whitespace to satisfy jcheck
 - Clean up code
   
   Removes stale FIXMEs, TODOs and comments; downgrades FIXMEs to TODOs where possible; wraps overly long lines.
 - Merge branch 'master' into 8266666
 - Merge branch 'master' into 8266666
 - Refactor SnippetTaglet
   
   Improves code style; reformats.
 - ... and 27 more: https://git.openjdk.java.net/jdk/compare/4b3a4fff...edb7cf85

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Changes: https://git.openjdk.java.net/jdk/pull/4795/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=24
  Stats: 5208 lines in 43 files changed: 5183 ins; 4 del; 21 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4795.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4795/head:pull/4795

PR: https://git.openjdk.java.net/jdk/pull/4795