RFR: 8266666: Implementation for snippets [v22]

[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 31 commits:

 - 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.
 - Improve snippet attributes parsing
   
   DocCommentParser now recognizes end-of-input better. To test that in testBadTagSyntax, the ToolBox.writeJavaFiles approach was used instead that of ClassBuilder.write. This is because ClassBuilder muddled doc comments with extra formatting such as trailing newline. So from within `ClassBuilder.MethodBuilder.setComments(java.lang.String...)` one couldn't make an immediate end-of-input: there was always `\n` appended to the comment.
   
   Also reformatted testBadTagSyntax for ease of comparing input and expected error output.
 - Merge branch 'master' into 8266666
 - Be more specific when testing for syntax errors
   
   This commit (i) improves tests for bad syntax, (ii) re-formats testBadTagSyntax for ease of commenting out individual testcases, and (iii) removes commented out testcases that are unfeasible.
 - Merge branch 'master' into 8266666
 - Merge branch 'master' into 8266666
 - Clean up tag parsing
   
   Removes two methods from an older implementation where HTML and javadoc tag attributes were modelled by different DocTree subtypes. Moves the `tagAttrs` method to TagParser responsible for parsing `@snippet` because `:` attribute terminator is specific to `@snippet`. Makes parser stop on `:`. Removes chatty discussion comments from code as suggested by Jonathan Gibbons.
 - ... and 21 more: https://git.openjdk.java.net/jdk/compare/f6cc1732...a2173165

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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=21
  Stats: 4932 lines in 44 files changed: 4906 ins; 4 del; 22 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