Add !var to use value of variable as value

app/Engine/CfnppFunctions.php

@@ -6,6 +6,7 @@ namespace App\Engine;
 
 use App\Dom\Document;
 use App\Dom\Node;
+use App\Dom\NodeValue;
 use App\Dom\NodeFunction;
 use App\Dom\NodeFunctionValue;
 
@@ -30,6 +31,8 @@ class CfnppFunctions
 
 	public function __construct(Cfnpp $compiler, CfnppOptions $options)
 	{
+		$this->compiler = $compiler;
+		$this->options = $options;
 	}
 
 	/**
@@ -92,4 +95,17 @@ class CfnppFunctions
 	{
 		return null;
 	}
+
+	/**
+	 * Get the value of a variable.
+	 *
+	 * @param  Node         $node
+	 * @param  NodeFunction $function
+	 * @return ?Node
+	 */
+	public function f_var(Node $node, NodeFunction $function): ?Node
+	{
+		$value = $this->options->getVariable($function->getValue());
+		return new NodeValue(null, $node->hasName() ? $node->getName() : null, $value);
+	}
 }