Make the web-ext config an ES module

.eslintrc.js

@@ -240,7 +240,7 @@ module.exports = {
       },
     },
     {
-      files: ["rollup.config.js"],
+      files: ["rollup.config.js", "web-ext-config.mjs"],
       parserOptions: {
         sourceType: "module",
       },

scripts/build-post.ts

@@ -58,7 +58,7 @@ async function makeSourceCodeBundle(): Promise<void> {
     "project.config.ts",
     "rollup.config.js",
     "tsconfig.json",
-    "web-ext-config.cjs",
+    "web-ext-config.mjs",
   ].map((file) => path.join(BASE_DIR, file));
 
   const dirs = ["@types", "docs", "patches", "scripts", "src"];

tsconfig.json

@@ -25,6 +25,7 @@
   },
   "exclude": [
     "compiled*",
-    "html"
+    "html",
+    "web-ext-config.mjs"
   ]
 }

web-ext-config.mjs

@@ -0,0 +1,28 @@
+import { createRequire } from "module";
+
+// This config needs to be an ES module. web-ext loads it with `import()`, and on
+// newer Node versions importing a `.cjs` file also exposes a `module.exports`
+// named export, which web-ext then rejects: `The config option "module.exports"
+// must be specified in camel case`. Reading project.config.ts still needs a
+// CommonJS `require` (so the sucrase-node hook kicks in), hence createRequire.
+const require = createRequire(import.meta.url);
+
+const optionalRequire = require("optional-require")(require);
+
+const config = require("./project.config").default;
+
+const customConfig = optionalRequire("./custom.config.cjs") || {};
+
+export default {
+  sourceDir: config.compiled,
+  artifactsDir: config.dist,
+  ignoreFiles: config.webextIgnoreFiles,
+  build: {
+    overwriteDest: true,
+  },
+  lint: {
+    warningsAsErrors: true,
+  },
+  run: customConfig.run || {},
+  sign: customConfig.sign || {},
+};