fix: make AdminForth instance single, also export global.adminforth so any code can use it at global level, adminforth instance is single in node environment

Author: ivictbor

adminforth/index.ts

@@ -125,6 +125,10 @@ class AdminForth implements IAdminForth {
   }
 
   constructor(config: AdminForthInputConfig) {
+    if (global.adminforth) {
+      throw new Error('AdminForth instance already created in this process. '+
+        'If you want to use multiple instances, consider using different process for each instance');
+    }
     this.codeInjector = new CodeInjector(this);
     this.configValidator = new ConfigValidator(this, config);
     this.restApi = new AdminForthRestAPI(this);