Alternate block header syntax

[robinlioret]

Address #2491

I'm facing the same issue with VSCode auto formatter. It's forcing me to disable formatOnSave, which is not a thing I want.

This PR adds an alternative syntax to the way blocks' attributes header are processed.

It is:

• Completely retrocompatible, so existing project don't have to modify anything.
• Simple yet unlikely to be a content

Per example:

/// admonition | Some title
@type: warning

Some content
///