feat: add support for show-source

Close #42

Author: javier-godoy

src/main/resources/META-INF/resources/frontend/code-viewer.ts

@@ -244,7 +244,10 @@ pre[class*="language-"] {
   }
 
   cleanupCode(text: string) : string {
-    return text.split('\n').filter(line=> 
+    return text.split('\n').map(line=>{
+        let m= line!.match("^(?<spaces>\\s*)//\\s*show-source\\s(?<line>.*)");
+        return m?m.groups!.spaces+m.groups!.line : line!;
+    }).filter(line=>
        !line.match("//\\s*hide-source(\\s|$)")
     && !line.startsWith('@Route')
     && !line.startsWith('@PageTitle')

src/test/java/com/flowingcode/vaadin/addons/demo/SampleDemoDefault.java

@@ -31,6 +31,7 @@ public class SampleDemoDefault extends Div {
 
   public SampleDemoDefault() {
     add(new Span("Demo component with defaulted @DemoSource annotation"));
+    // show-source System.out.println("this line will be displayed in the code snippet");
     this.getClass(); // hide-source (this line will not be displayed in the code snippet)
   }
 }