feat: add symbols to reference certain private properties on LogMessa…

…ge class

BREAKING CHANGE: Previously you could directly access the private properties of LambdaLog. In order to add some integrity, they are no longer using standard property names starting with an underscore and are instead referenced using symbols instead. For advanced usage, these symbols are exported as a static property on the LambdaLog class under `LambdaLog.symbols`.

lib/LambdaLog.js

@@ -1,6 +1,10 @@
 const EventEmitter = require('events');
 const LogMessage = require('./LogMessage');
 
+const symbols = {
+  LEVELS: Symbol('levels')
+};
+
 /**
  * @external EventEmitter
  * @see https://nodejs.org/api/events.html#events_class_eventemitter
@@ -221,4 +225,6 @@ class LambdaLog extends EventEmitter {
   }
 }
 
+LambdaLog.symbols = symbols;
+
 module.exports = LambdaLog;