chore: use --bundleConfigAsCjs to load rollup config

Ideally, we'd like to use a `mjs` config, but that necessitates an import type assertion for the import of `package.json`,
and this feature is not supported by node v14.
So we'll be able to use a `rollup.config.mjs` config only when we'll stop supporting node v14.
See #1815

package.json

@@ -75,8 +75,8 @@
     "test:watch": "vitest --watch",
     "test:build": "vitest --mode test-build",
     "tsd": "tsc -p test-dts/tsconfig.tsd.json",
-    "build": "rollup -c rollup.config.ts",
-    "prepare": "rollup -c rollup.config.ts",
+    "build": "rollup -c rollup.config.ts --bundleConfigAsCjs",
+    "prepare": "rollup -c rollup.config.ts --bundleConfigAsCjs",
     "lint": "eslint --ext .ts src/ tests/",
     "lint:fix": "pnpm run lint --fix",
     "docs:dev": "vitepress dev docs",