fix(constants): add 'as const' to FIRST constant for type safety

Author: bhovhannes

src/constants.ts

@@ -46,7 +46,7 @@ export const ORDERDOT = '_';
  * @readonly
  * @type {String}
  */
-export const FIRST = "$$FIRST";
+export const FIRST = "$$FIRST" as const;
 
 /**
  * Key used to specify a limit on the number of results. If this key is present, the value is used.<br/>