RFR: 8266666: Implementation for snippets [v5]

[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 two additional commits since the last revision:

 - Change AnnotatedText<T> to StyledText
   
   Renames the class, removes its generic parameter, and changes the related terminology from "annotate" to "style".
 - Restructure ...toolkit.taglets.snippet.** packages
   
   This commit moves the contents of the jdk.javadoc.internal.doclets.toolkit.taglets.snippet.{action,parser,text} packages into the jdk.javadoc.internal.doclets.toolkit.taglets.snippet package.

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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/34b274cb..01afad63

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=03-04

  Stats: 1087 lines in 18 files changed: 513 ins; 540 del; 34 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