Provide folding range for sibling attributes

[apupier]

part of #719

basic use case covered, other cases potentially to cover:

• include comment lines
• include emtpy lines

MoreThan2AttributesFoldingDemo.mp4

[ggrossetie]

I think the only uncovered use case is a multi-line attribute:

= Document
:description: If you have a very long line of text \
that you need to substitute regularly in a document, \
you may find it easier to split the value neatly in the header \
so it remains readable to folks looking at the AsciiDoc source.
:attr2: value
:attr3: value

content

[ggrossetie]

1. include comment lines

I don't know about that. I think a comment is an indication that attributes are "grouped":

= Document
// urls
:url-foo: foo
:url-bar: bar
// built-in
:toc:
:author: Aurélien

content

We might want to fold all document attributes at once but I think we should also allow to fold each "group".

2. include empty lines

It's important to note that:

The document header may not contain empty lines. The first empty line the processor encounters after the document header begins marks the end of the document header and the start of the document body.

https://docs.asciidoctor.org/asciidoc/latest/document/header/#document-header-structure

It might be misleading if we fold all attributes in the document header:

= Document
:doc-attr1: value
:doc-attr2: value

:not-doc-attr: value

content

For attributes defined in the body I think it's fine:

= Document
:attr1: value
:attr2: value

content

:body-attr1: value
:body-attr2: value

:body-attr3: value
:body-attr4: value

more content

But again it makes sense to fold per "group". I think that's the whole purpose of adding an empty line, to visually create two groups of attributes.

Anyway, we can discuss that in follow-up issues.