runtime(java): Fold multi-line comments with the syntax kind of &fdm (#15016)

runtime(java): Fold multi-line comments with the syntax kind of &fdm (#15016)

Also:

- Restore the capability to mark as an error braces nested
  in parens with g:javaInParen.
- Try not to fold top-level-type bodies.  (Defining multiple
  package-private top level types in a single source file is
  not recommended as it can impose order among compilation
  units; so it is assumed that only one such top level type
  is usually defined.)
- Compose ‘method header’ highlighting and block braces
  folding.
- Do not highlight block braces whenever ‘method header’
  highlighting is requested.

This bundling of ‘method headers’ and block braces for
highlighting can be traced back to Vim v5.0; however, no
comment or documentation entry conveys any justification.
For example, it is hard to discover the connection between
block braces for "while", "if", etc., statements and method
body block braces.  The former behaviour can be attained in,
e.g. ~/.vim/after/syntax/java.vim:

------------------------------------------------------------
if exists("g:java_highlight_functions")
    syn clear javaBlock javaInParen
    syn match javaBlockOther "[{}]"
    syn region javaBlock transparent matchgroup=javaBlockStart
	\ start="\%(^\|^\S[^:]\+\)\@120<!{" end="}" fold
    hi def link javaBlockStart javaFuncDef
    hi def link javaBlockOther javaBlockStart

    if exists("g:java_mark_braces_in_parens_as_errors")
	syn match javaInParen contained "[{}]"
    endif
endif
------------------------------------------------------------

Note: Read ‘a method header omitting a _throws_ clause’ for
every ‘method header’ appellation used above.

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