Parse arguments for pseudo-class functions

so that the AST doesn't just contain tokens. Previously the arguments
were something like

```
[(ident-token "div"),
 (comma-token),
 (whitespace-token " "),
 (ident-token "span"),
 (delim-token "."),
 (ident-token "wide"),
 (comma-token),
 (whitespace-token " "),
 (delim-token "."),
 (ident-token "hidden")]
```

Now those tokens will be represented in the AST as

```
[(type-selector (wqname (ident-token "div"))),
 (compound-selector
   (type (type-selector (wqname (ident-token "span"))))
   (subclasses
     (class-selector (ident-token "wide"))
   )
   (pseudo-elements)
 ),
 (class-selector (ident-token "hidden"))]
```

lib/syntax_tree/css/selectors.rb

@@ -509,7 +509,8 @@ def pseudo_class_selector
           PseudoClassSelector.new(value: consume(IdentToken))
         in Function
           node = consume(Function)
-          function = PseudoClassFunction.new(name: node.name, arguments: node.value)
+          arguments = Selectors.new(node.value).parse
+          function = PseudoClassFunction.new(name: node.name, arguments: arguments)
           PseudoClassSelector.new(value: function)
         else
           raise MissingTokenError, "Expected pseudo class selector to produce something"

test/selectors_test.rb

@@ -82,7 +82,7 @@ class SelectorsTest < Minitest::Spec
           end
         end
 
-        it "parses a compound selector with a pseudo-class" do
+        it "parses a compound selector with a pseudo-class selector" do
           actual = parse_selectors("div.flex:hover")
 
           assert_pattern do
@@ -98,6 +98,33 @@ class SelectorsTest < Minitest::Spec
           end
         end
 
+        it "parses a compound selector with a pseudo-class function" do
+          actual = parse_selectors(".flex:not(div, span.wide, .hidden)")
+
+          assert_pattern do
+            actual => [
+              Selectors::CompoundSelector[
+                type: nil,
+                subclasses: [
+                  Selectors::ClassSelector[value: { value: "flex" }],
+                  Selectors::PseudoClassSelector[
+                    value: Selectors::PseudoClassFunction[
+                      name: "not",
+                      arguments: [
+                        Selectors::TypeSelector[value: { name: { value: "div" } }],
+                        Selectors::CompoundSelector[
+                          Selectors::TypeSelector[value: { name: { value: "span" } }],
+                          Selectors::ClassSelector[value: { value: "wide" }],
+                        ],
+                      ],
+                    ],
+                  ],
+                ],
+              ]
+            ]
+          end
+        end
+
         it "parses a compound selector with pseudo-elements and pseudo-classes" do
           actual = parse_selectors("div.flex:hover::first-line:last-child:active::first-letter")
 
@@ -208,7 +235,6 @@ class SelectorsTest < Minitest::Spec
             ]
           end
         end
-
       end
 
       describe "formatting" do