Define the reference syntax

[jeluard]

References point to extensions elements. Make sure the syntax is unambiguous regarding:

• local vars usage (as introduced by let blocks)
• current extensions elements
• current platform elements
• elements defined in remote extensions

[jeluard]

Semantic adopted is:

@views/pluto.id

where views defines the element type, pluto (namespace) the optional source of the element (can reference dependencies) and id the element id.