LibWeb: Implement the document title attribute closer to the spec

The main differences between our current implementation and the spec
are:

    * The title element need not be a child of the head element.

    * If the title element does not exist, the default value should be
      the empty string - we currently return a null string.

    * We've since added AOs for several of the spec steps here, so we
      do not need to implement those steps inline.

Tests/LibWeb/Text/expected/title.txt

@@ -0,0 +1,14 @@
+1: ""
+2a: 0
+2b: 1
+2c: "This is a title!"
+2d: "This is a title!"
+3: ""
+4a: 3
+4b: ""
+4c: ""
+4d: ""
+4e: 3
+4f: "This is another title!"
+4g: ""
+4h: ""

Tests/LibWeb/Text/input/title.html

@@ -0,0 +1,42 @@
+<script src="include.js"></script>
+<script>
+    test(() => {
+        // The title is the empty string by default.
+        println(`1: "${document.title}"`);
+
+        // When the title is set, and a title element does not exist, one is added to to head element.
+        let titleElements = document.getElementsByTagName('title');
+        println(`2a: ${titleElements.length}`)
+
+        document.title = 'This is a title!';
+
+        titleElements = document.getElementsByTagName('title');
+        println(`2b: ${titleElements.length}`)
+        println(`2c: "${titleElements[0].innerText}"`);
+        println(`2d: "${document.title}"`);
+
+        // Removing the title element sets the title back to default.
+        titleElements[0].remove();
+        println(`3: "${document.title}"`);
+
+        // After adding several title elements to the body, setting the title updates the text
+        // content of only the first title element.
+        document.body.appendChild(document.createElement('title'));
+        document.body.appendChild(document.createElement('title'));
+        document.body.appendChild(document.createElement('title'));
+
+        titleElements = document.getElementsByTagName('title');
+        println(`4a: ${titleElements.length}`)
+        println(`4b: "${titleElements[0].innerText}"`);
+        println(`4c: "${titleElements[1].innerText}"`);
+        println(`4d: "${titleElements[2].innerText}"`);
+
+        document.title = 'This is another title!';
+
+        titleElements = document.getElementsByTagName('title');
+        println(`4e: ${titleElements.length}`)
+        println(`4f: "${titleElements[0].innerText}"`);
+        println(`4g: "${titleElements[1].innerText}"`);
+        println(`4h: "${titleElements[2].innerText}"`);
+    });
+</script>