Add explicit static keyword for nested classes that do not depend on outer context

Author: miasma

commonmark/src/test/java/org/commonmark/test/UsageExampleTest.java

@@ -96,7 +96,7 @@ public void customizeRendering() {
         assertThat(renderer.render(document)).isEqualTo("<p>Example:</p>\n<pre>code\n</pre>\n");
     }
 
-    class WordCountVisitor extends AbstractVisitor {
+    static class WordCountVisitor extends AbstractVisitor {
 
         int wordCount = 0;
 
@@ -112,7 +112,7 @@ public void visit(Text text) {
         }
     }
 
-    class ImageAttributeProvider implements AttributeProvider {
+    static class ImageAttributeProvider implements AttributeProvider {
         @Override
         public void setAttributes(Node node, String tagName, Map<String, String> attributes) {
             if (node instanceof Image) {
@@ -121,7 +121,7 @@ public void setAttributes(Node node, String tagName, Map<String, String> attribu
         }
     }
 
-    class IndentedCodeBlockNodeRenderer implements NodeRenderer {
+    static class IndentedCodeBlockNodeRenderer implements NodeRenderer {
 
         private final HtmlWriter html;