Migrates ESLint configuration to new (9+) "flat format"

.eslint.config.mjs

@@ -0,0 +1,36 @@
+// Thunderbird User.JS ESLint configuration file
+
+import js from "@eslint/js";
+
+export default [
+    js.configs.recommended,
+    {
+        rules: {
+            // Expect only double-quoted strings.
+            quotes: ["error", "double"],
+
+            // Expect a semicolon after each statement.
+            semi: ["error", "always", {"omitLastInOneLineBlock": false}],
+
+            // As project code style, don't allow tabulation nor trailing whitespaces.
+            "no-tabs": "error",
+            "no-trailing-spaces": "error",
+
+            // Don't allow whitespace before semicolons.
+            "semi-spacing": ["error", {"before": false}],
+
+            // Don't allow irregular whitespace characters in our sheet.
+            "no-irregular-whitespace": ["error", {"skipStrings": false, "skipComments": false}],
+        },
+        languageOptions: {
+            // From <https://searchfox.org/mozilla-central/rev/c938c7416c633639a5c8ce4412be586eefb48005/modules/libpref/parser/src/lib.rs#296>
+            globals: {
+                pref: "readonly",
+                user_pref: "readonly",
+                sticky: "readonly",
+                locked: "readonly",
+                sticky_pref: "readonly",
+            },
+        },
+    },
+];

.eslintrc.yml

@@ -1,45 +0,0 @@
-%YAML 1.2
----
-
-root: true
-
-extends: 'eslint:recommended'
-
-globals:
-  # From <https://searchfox.org/mozilla-central/rev/c938c7416c633639a5c8ce4412be586eefb48005/modules/libpref/parser/src/lib.rs#296>
-  pref: true
-  user_pref: true
-  sticky: true
-  locked: true
-  sticky_pref: true
-
-rules:
-  # Expect a semicolon after each statement.
-  semi:
-    - "error"
-    - "always"
-    -
-      omitLastInOneLineBlock: false
-
-  # As internal code style, don't allow tabulation.
-  no-tabs: "error"
-  # ... nor trailing spaces !
-  no-trailing-spaces: "error"
-
-  # Expect only double-quoted strings.
-  quotes:
-    - "error"
-    - "double"
-
-  # Don't allow whitespace before semicolons.
-  semi-spacing:
-    - "error"
-    -
-      before: false
-
-  # Don't allow irregular whitespace characters in our sheet.
-  no-irregular-whitespace:
-    - "error"
-    -
-      skipStrings: false
-      skipComments: false

.github/workflows/linting.yml

@@ -7,13 +7,11 @@ jobs:
   build:
     name: Run ESLint on user.js
     runs-on: ubuntu-latest
-    env:
-      ESLINT_USE_FLAT_CONFIG: false
 
     steps:
       - uses: actions/checkout@v3
 
       - uses: actions/setup-node@v3
 
-      - run: npm install -g eslint
-      - run: eslint user.js
+      - run: npm install eslint @eslint/js
+      - run: npx eslint -c .eslint.config.mjs user.js