[marcoroth]

This pull request adds LightningCSS.parse, which returns the AST Lightning CSS parsed a stylesheet to, along with LightningCSS::Node and LightningCSS::Visitor to walk it.

The API follows what oxc-ruby does for JavaScript.

parsed = LightningCSS.parse(".card .title:hover { color: #f00 }")

parsed.stylesheet   #=> the AST exactly as Lightning CSS serializes it
parsed.warnings     #=> []

root = parsed.root

root.type                    
#=> "stylesheet"

root.every("declaration")    
#=> every declaration in the stylesheet

root.rules.first.value.selectors.first.map(&:type)
#=> ["class", "combinator", "class", "pseudo-class"]

Resolves #11