use ACTIONS_STEP_DEBUG to determine debug mode, use isTrueString lib function

src/action.js

@@ -1,5 +1,6 @@
 import {collect_all} from './index';
+import {isTrueString} from './lib';
 
-const debug = process.env['ACTIONS_STEP_DEBUG'] === '1' || process.env['ACTIONS_STEP_DEBUG'] === 'true';
+const debug = isTrueString(process.env['ACTIONS_STEP_DEBUG']);
 
 collect_all(debug, true);

src/lib.js

@@ -14,3 +14,10 @@ export function result_to_output(results) {
     }
   }
 }
+
+export function isTrueString(str) {
+  return str === '1'
+    || str === 'true'
+    || str === 'True'
+    || str === 'TRUE';
+}