feat: define DOM

Sources/TreeConstructor/Document.swift

@@ -0,0 +1,5 @@
+public struct Document: ~Copyable, Sendable {
+    public var title: String
+    public var body: Optional<Node>
+    public var head: Optional<Node>
+}

Sources/TreeConstructor/HTMLElement.swift

@@ -0,0 +1,3 @@
+public struct HTMLElement: Sendable {
+    public var tagName: String
+}

Sources/TreeConstructor/Node.swift

@@ -0,0 +1,9 @@
+public struct Node: Sendable {
+    public var value: NodeValue
+    public var childNodes: [Self]
+}
+
+public enum NodeValue: Sendable {
+    case element(HTMLElement)
+    case text(Text)
+}

Sources/TreeConstructor/Text.swift

@@ -0,0 +1,3 @@
+public struct Text: Sendable {
+    public var data: String
+}

Sources/TreeConstructor/TreeConstructor.swift

@@ -1,9 +1,11 @@
 public import Tokenizer
 
 public struct TreeConstructor: ~Copyable {
+    public var document: Document
     private var mode: InsertionMode
 
     public init() {
+        self.document = .init(title: "", body: nil, head: nil)
         self.mode = .initial
     }
 }