RFR: 8266666: Implementation for snippets [v19]

[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:

  Recover from a sloppy merge
  
  Apparently, I've performed at least one sloppy merge while developing the feature in sandbox. As a result, some changes brought by e4d045402fa1992a1d91586bd4f67362d07f543c were discarded. The specdiff for the CSR actually shows this and has to be regenerated.

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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/87e765b0..0d9c53a6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=18
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=17-18

  Stats: 6 lines in 2 files changed: 4 ins; 0 del; 2 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