config: append, not overwrite RULES_DIR

Author: denis-sokolov

src/config/index.ts

@@ -7,10 +7,17 @@ import { formattingRules } from "./formatting";
 import { overrides } from "./overrides";
 import { otherRules } from "./other";
 
-rulesDirPlugin.RULES_DIR = [
+// Merge, not overwrite.
+// Otherwise in some setups the order of file loading is wrong and
+// we overwrite RULES_DIR configured elsewhere.
+const previousRulesDir =
+  typeof rulesDirPlugin.RULES_DIR === "string"
+    ? [rulesDirPlugin.RULES_DIR]
+    : (rulesDirPlugin.RULES_DIR ?? []);
+rulesDirPlugin.RULES_DIR = previousRulesDir.concat([
   // Relative to the individual project’s root
   ".eslint/custom-rules",
-];
+]);
 
 export const main = {
   env: {