runtime(java): Manage circularity for every :syn-included syntax file

runtime(java): Manage circularity for every :syn-included syntax file

With "g:markdown_fenced_languages" defined and "java" added
to its list, a circular dependency between the Markdown and
Java syntax files will be made.  To break it, no Markdown
documentation comments will be recognised in fenced blocks
in Markdown files; in order to view Java source files,
"java" must be removed from "g:markdown_fenced_languages",
and this task can be automated as follows.

1) Add to "~/.after/ftplugin/java.vim":
------------------------------------------------------------
if exists("g:markdown_fenced_languages") &&
	\ !(exists("g:java_ignore_javadoc") ||
	\ exists("g:java_ignore_markdown"))
    let s:idx = index(g:markdown_fenced_languages, 'java')
    if s:idx > -1
	call remove(g:markdown_fenced_languages, s:idx)
    endif
    unlet s:idx
endif
------------------------------------------------------------

2) Optionally add to "~/.after/ftplugin/markdown.vim":
------------------------------------------------------------
if exists("g:markdown_fenced_languages") &&
	\ index(g:markdown_fenced_languages, 'java') < 0
    call add(g:markdown_fenced_languages, 'java')
endif
------------------------------------------------------------

(Make sure that the above snippets appear in the files under
the "ftplugin" NOT "syntax" directory.)

Finally, unless the new version of the syntax file is made
available from "$VIMRUNTIME" (and from "~/.vim/syntax" if
necessary), OTHER discoverable file versions will be used
whose behaviour may interfere with this fix.

related: #15740
closes: #15796

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>