Fix 2699 : filename set at parsing stage + nodes.filename scope + filename in the Root AST node

[hl037]

What:

• Scope node.filename changes so that it's easier to follow how this global variable is changed
• Root AST node call its parent to get a filename too
• Visitors gets their this.filename from the root node if not provided in the options

Why:

I am implementing a loader to retrieve all the variables defined in a stylus file (and its imports). The easiest way to do it is to look into Evaluator.global.scope. However, the setup adds builtin globals and filtering is necessary. The best way to filter is to check if the AST node's filename is null. However, the parser does not set the filename of the ast nodes.

fix #2699

While implementing this, I encountered several problems with the nodes.filename global variable :

• It's hard to track when and if it is restored
• It can leak to other tests

This is why I added code to precisely mark the scope of the modification, and restore its previous value then.

How:

See commit changes (it's quite short)

Checklist:

• Documentation N/A
• Unit Tests
• Code complete
• Changelog

I am waiting your review before writing changelogs etc.