fix: add CJS support to @netlify/runtime-utils to fix ERR_REQUIRE_ESM

[serhalp]

Problem

Fixes #437

When using @netlify/blobs in a CJS serverless function, the deployed function fails at runtime with ERR_REQUIRE_ESM:

Error [ERR_REQUIRE_ESM]: require() of ES Module /var/task/node_modules/@netlify/runtime-utils/dist/main.js 
from /var/task/node_modules/@netlify/blobs/dist/main.cjs not supported.

Root Cause

• @netlify/blobs is built in both CJS and ESM formats
• The CJS build contains require('@netlify/runtime-utils') statements
• @netlify/runtime-utils was configured to export ESM-only
• Node.js throws ERR_REQUIRE_ESM when trying to require() an ESM-only module from a CJS context

Solution

Make @netlify/runtime-utils dual-format (CJS + ESM) so it can be consumed by both module systems.

Changes

1. packages/runtime-utils/tsup.config.ts: Updated to build both cjs and esm formats
2. packages/runtime-utils/package.json: Added proper conditional exports for both formats

Future Work

When @netlify/blobs becomes ESM-only, we can remove CJS support from @netlify/runtime-utils as well.

Testing

✅ CJS build works:

node -e "const { base64Encode } = require('./packages/runtime-utils/dist/main.cjs'); console.log(base64Encode('test'))"
# Output: dGVzdA==

✅ ESM build still works:

node --input-type=module -e "import { base64Encode } from './packages/runtime-utils/dist/main.js'; console.log(base64Encode('test'))"
# Output: dGVzdA==

✅ @netlify/blobs CJS build loads successfully:

node -e "const blobs = require('./packages/blobs/dist/main.cjs'); console.log(typeof blobs.getStore)"
# Output: function