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",
+            },
+        },
+    },
+];