js: support writing debugger statement in JSBody

Fix #974

core/src/main/java/org/teavm/backend/javascript/rendering/AstWriter.java

@@ -291,6 +291,9 @@ public class AstWriter {
             case Token.WHILE:
                 print((WhileLoop) node);
                 break;
+            case Token.DEBUGGER:
+                writer.append("debugger;");
+                break;
             default:
                 if (node instanceof InfixExpression) {
                     printInfix((InfixExpression) node, precedence);

core/src/test/java/org/teavm/backend/javascript/rendering/AstWriterTest.java

@@ -255,6 +255,11 @@ public class AstWriterTest {
         assertEquals("delete a.b;", transform("delete a.b;"));
     }
 
+    @Test
+    public void writesDebugger() throws IOException {
+        assertEquals("debugger;debugger;", transform("debugger;  debugger;"));
+    }
+
     @Test
     public void writesGlobalRef() throws IOException {
         assertEquals(