RFR: JDK-8223805: DocCommentParser should allow for <main> and </main>

[Jonathan Gibbons]

Please review a relatively simple upgrade to the DocCommentParser for 
handling standalone HTML files.

In a standalone file, HTML content is treated as being in 3 parts ... 
the preamble, the  body, and the postamble, where the body is the 
content of an equivalent doc comment.  Traditionally, the preamble ends 
at the end of the opening tag for the `body` element, and the body ends 
at the beginning of the start of the closing tag for the `body` element. 
In other words, the body has traditionally been the inner HTML of the 
enclosing `body` element.

Since then, a style has evolved where authors are wrapping the content 
in a `main` element as well, presumably to satisfy accessibility 
checkers (which is good).  But this conflicts with the traditional 
determination of the content of the file, because (amongst other 
reasons) there can only be one `main` element in a generated file.

The change is for the preamble to also include the opening tag of a 
`main` element if it immediately follows the opening tag of the `body` 
element (allowing for inter-element whitespace. The change is also for 
the body to stop at the closing tag of a `main` element if one is 
encountered.

Two test cases are added to an existing test. The test cases consist of 
HTML files containing `main` elements as well as `body` elements. The 
corresponding .out files are dumps of the doc comment tree, showing the 
content of the preamble, body and postamble.

The change was also testing by building JDK API docs, running doccheck, 
and reviewing the results, to confirm that affected files which 
previously contained errors no longer do so.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8223805
Webrev: 
file:///w/jjg/work/jdk.closed.dev/8223805/webrev.00/webrev/index.html