Introduce Oxc::Node and Oxc::Visitor

[marcoroth]

This pull request gives parse a way to be walked. Until now it only returned a JSON blob with no way of traversing it.

root = Oxc.parse(source).root

root.every("Identifier")   # every node of a type
root.at(offset)            # the innermost node covering a byte offset
root.each                  # an Enumerator, and Enumerable, so find and count work
root.ancestors             # what it sits inside
root.slice(source)         # its own source back

A field reads by name, and comes back wrapped when it holds a node, so reads chain.

declaration.kind                          #=> "let"
declaration.declarations.first.id["name"] #=> "count"

respond_to_missing? is wired to the real keys, so a field the node does not carry raises NoMethodError.

Oxc::Visitor answers a node with the method named after its type, and walks through anything nothing answers.

class Reads < Oxc::Visitor
  def visit_assignment_expression(node)
    puts "#{node.left["name"]} #{node.operator}"

    visit_children(node)
  end
end

Reads.new.visit(Oxc.parse(source))

Dispatch snake-cases the ESTree type with acronym runs handled, so TSTypeAnnotation answers to visit_ts_type_annotation and JSXElement to visit_jsx_element.