feat: improve content state management

Improve content state management by adding a state change listener and
storing the state as an internal variable
Fixes #4

Author: mlopezFC

src/main/java/com/flowingcode/vaadin/addons/markdown/BaseMarkdownComponent.java

@@ -23,6 +23,7 @@
 import com.vaadin.flow.component.dependency.CssImport;
 import com.vaadin.flow.component.dependency.NpmPackage;
 import com.vaadin.flow.component.react.ReactAdapterComponent;
+import com.vaadin.flow.function.SerializableConsumer;
 
 /**
  * Base class for Markdown based Components.
@@ -47,13 +48,16 @@ public class BaseMarkdownComponent extends ReactAdapterComponent implements HasS
    */
   public enum DataColorMode {DARK,LIGTH,AUTO};
 
+  private String content;
+  
   /**
    * Base constructor that receives the content of the markdown component.
    * 
    * @param content content to be used in the Markdown component
    */
   public BaseMarkdownComponent(String content) {
     setContent(content);
+    this.addContentChangeListener(c->this.content = c);
   }
 
   /**
@@ -62,7 +66,7 @@ public BaseMarkdownComponent(String content) {
    * @return the content of the Markdown component
    */
   public String getContent() {
-    return getState("content", String.class);
+    return content;
   }
 
   /**
@@ -71,9 +75,19 @@ public String getContent() {
    * @param content retrieved from the state of the component
    */
   public void setContent(String content) {
+    this.content = content;
     setState("content", content);
   }
 
+  /**
+   * Adds the specified listener for the content change event.
+   * 
+   * @param listener the listener callback for receiving content changes
+   */
+  public void addContentChangeListener(SerializableConsumer<String> listener) {
+    addStateChangeListener("content", String.class, listener);
+  }
+  
   /**
    * Sets the color mode of the Markdown component.
    *