runtime(java): Recognise the CommonMark form (///) of Javadoc comments

runtime(java): Recognise the CommonMark form (///) of Javadoc comments

Complement "g:java_ignore_javadoc" with "g:java_ignore_html"
and "g:java_ignore_markdown" to allow selectively disabling
the recognition of HTML and CommonMark respectively.

(Note that this is not a preview feature.)

======================== LIMITATION ========================

According to the syntactical details of JEP 467:

> Any leading whitespace and the three initial / characters
> are removed from each line.
>
> The lines are shifted left, by removing leading whitespace
> characters, until the non-blank line with the least
> leading whitespace has no remaining leading whitespace.
>
> Additional leading whitespace and any trailing whitespace
> in each line is preserved, because it may be significant.

the following example:
------------------------------------------------------------
///    A summary sentence.
///     A list:
///      - Item A.
///     - Item B.
///
///     Some code span, starting here `
///      1 + 2 ` and ending at the previous \`.
------------------------------------------------------------

should be interpreted as if it were written thus:
------------------------------------------------------------
///A summary sentence.
/// A list:
///  - Item A.
/// - Item B.
///
/// Some code span, starting here `
///  1 + 2 ` and ending at the previous \`.
------------------------------------------------------------

Since automatic line rewriting will not be pursued, parts of
such comments having significant whitespace may be ‘wrongly’
highlighted.  For convenience, a &fex function is defined to
‘correct’ it: g:javaformat#RemoveCommonMarkdownWhitespace()
(:help ft-java-plugin).

References:
https://openjdk.org/jeps/467
https://spec.commonmark.org/0.31.2

closes: #15740

Co-authored-by: Tim Pope <code@tpope.net>
Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>