Add if-no-files-found option to merge action

src/shared/constants.ts

@@ -0,0 +1,16 @@
+export enum NoFileOptions {
+  /**
+   * Default. Output a warning but do not fail the action
+   */
+  warn = 'warn',
+
+  /**
+   * Fail the action with an error message
+   */
+  error = 'error',
+
+  /**
+   * Do not output any warnings or errors, the action does not fail
+   */
+  ignore = 'ignore'
+}