RFR: 8266666: Implementation for snippets [v24]

[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 incrementally with one additional commit since the last revision:

  Address feedback
  
  1. Reverts changes from CommentUtils.java (unused methods)
  2. Changes misguided regex-related assertion to tests

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4795/files
  - new: https://git.openjdk.java.net/jdk/pull/4795/files/3c654e6b..4a22c09d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=23
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=22-23

  Stats: 116 lines in 3 files changed: 93 ins; 22 del; 1 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