LibJS: Implement basic boolean coercion

We can't expect the conditionals in "if" and "while" statements to
always return a bool, so we need to know how to boolify a JS::Value.

Libraries/LibJS/Value.h

@@ -113,6 +113,7 @@ public:
     }
 
     String to_string() const;
+    bool to_boolean() const;
 
 private:
     Type m_type { Type::Undefined };