LibCpp: Generalize ASTNode::dump() to support redirecting its output

Previously, ASTNode::dump() used outln() for output, which meant it
always wrote its output to stdout.

After this commit, ASTNode::dump() receives an 'output' argument (which
is stdout by default). This enables writing the output to somewhere
else.

This will be useful for testing the LibCpp Parser with the output of
ASTNode::dump.

Userland/Utilities/cpp-parser.cpp

@@ -40,5 +40,5 @@ int main(int argc, char** argv)
         dbgln("{}", error);
     }
 
-    root->dump(0);
+    root->dump();
 }