Mark implicit macro argument default values as such with an attribute

This change causes no difference to compiled templates or to macro argument semantics.

Consider the following macro:
```twig
{% macro marco(po, lo = null) %}{% endmacro %}
```

With this change, the `ConstantExpression` for argument `po` will have an attribute `isImplicit`, whose value will be `true`. (Note that `lo` will not have that attribute.)

This allows node visitors to distinguish between arguments that do and those that do not have
explicit default values even if the value is `null`.

This is useful for [static code analysis](twigphp#4003).

For example, a static analysis tool might consider arguments with no explicit default value as non-optional.

src/ExpressionParser.php

@@ -591,7 +591,7 @@ public function parseFilterExpressionRaw($node, $tag = null)
      * Parses arguments.
      *
      * @param bool $namedArguments Whether to allow named arguments or not
-     * @param bool $definition     Whether we are parsing arguments for a function definition
+     * @param bool $definition     Whether we are parsing arguments for a function (or macro) definition
      *
      * @return Node
      *
@@ -642,6 +642,7 @@ public function parseArguments($namedArguments = false, $definition = false, $al
                 if (null === $name) {
                     $name = $value->getAttribute('name');
                     $value = new ConstantExpression(null, $this->parser->getCurrentToken()->getLine());
+                    $value->setAttribute('isImplicit', true);
                 }
                 $args[$name] = $value;
             } else {

tests/ParserTest.php

@@ -14,6 +14,7 @@
 use PHPUnit\Framework\TestCase;
 use Twig\Environment;
 use Twig\Error\SyntaxError;
+use Twig\Lexer;
 use Twig\Loader\LoaderInterface;
 use Twig\Node\Node;
 use Twig\Node\SetNode;
@@ -175,6 +176,37 @@ public function testGetVarName()
         $this->addToAssertionCount(1);
     }
 
+    public function testImplicitMacroArgumentDefaultValues()
+    {
+        $template = '{% macro marco (po, lo = null) %}{% endmacro %}';
+        $lexer = new Lexer(new Environment($this->createMock(LoaderInterface::class)));
+        $stream = $lexer->tokenize(new Source($template, 'index'));
+
+        $argumentNodes = $this->getParser()
+            ->parse($stream)
+            ->getNode('macros')
+            ->getNode('marco')
+            ->getNode('arguments')
+        ;
+
+        self::assertTrue(
+            $argumentNodes->getNode('po')->hasAttribute('isImplicit')
+        );
+        self::assertTrue(
+            $argumentNodes->getNode('po')->getAttribute('isImplicit')
+        );
+        self::assertNull(
+            $argumentNodes->getNode('po')->getAttribute('value')
+        );
+
+        self::assertFalse(
+            $argumentNodes->getNode('lo')->hasAttribute('isImplicit')
+        );
+        self::assertNull(
+            $argumentNodes->getNode('lo')->getAttribute('value')
+        );
+    }
+
     protected function getParser()
     {
         $parser = new Parser(new Environment($this->createMock(LoaderInterface::class)));