Add implementation for the length property for HTML Nodes

See:
https://dom.spec.whatwg.org/#concept-node-length
https://heycam.github.io/webidl/#dfn-code-unit

Sources/include/HTMLNode.h

@@ -137,6 +137,11 @@ typedef NS_ENUM(unsigned short, HTMLDocumentPosition)
  */
 @property (nonatomic, copy)	NSString *innerHTML;
 
+/**
+ The length of the node as described in https://dom.spec.whatwg.org/#concept-node-length
+ */
+@property (nonatomic, assign) NSUInteger length;
+
 /**
  @abstract Use concrete subclasses of the HTML Node.
  */
@@ -164,6 +169,13 @@ typedef NS_ENUM(unsigned short, HTMLDocumentPosition)
  */
 - (NSUInteger)childNodesCount;
 
+/**
+ Checks whether the node is empty as described in https://dom.spec.whatwg.org/#concept-node-length
+
+ @return `YES` if the node is empty, `NO` otherwise.
+ */
+- (BOOL)isEmpty;
+
 /**
  Returns the child node at a given index.