Fix codesandbox ci

[joshua-burbidge]

I was working on a fix for #1050 and may have a PR soon, but thought I would fix the CI first. It's been failing for the past 2 PRs for a few reasons: https://ci.codesandbox.io/status/final-form/react-final-form/pr/1047

• Deprecated dependency iltorb (transitive dependency of bundle-size) causing the yarn install to fail. replaced bundlesize with size-limit which does the same thing and is more actively maintained
  • Note: I increased the listed sizes, but that's only because the actual sizes are bigger than the previous listed thresholds, and previously bundlesize was not actually comparing against the threshold, because bundlesize uses maxSize as the property name, not threshold
• Config files were not playing nice with the node 18 used by the ci. needed to change file extensions to mjs so they are recognized as ES modules

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit bfc2308:

Sandbox	Source
React Final Form - Simple Example	Configuration
React Final Form - Synchronous Record Level Validation	Configuration
React Final Form - Synchronous Field Level Validation	Configuration
React Final Form - Submission Errors	Configuration
React Final Form - Subscriptions Example	Configuration

[joshua-burbidge]

Hi @erikras, can you review? Thanks 👍

[erikras-richard-agent]

Thanks for fixing the CI! 🙏 This PR addresses real issues:

What this fixes:
✅ Replaces deprecated bundlesize (with iltorb transitive dep) with modern size-limit
✅ Renames config files to .mjs for proper ES module support
✅ Updates ESLint config to handle .mjs files
✅ CodeSandbox CI now passing ✓

Concerns:
⚠️ Size limits increased significantly - from 2-3kB to 4kB across all bundles:

• UMD: 2kB → 4kB (+100%)
• ES: 3kB → 4kB (+33%)
• CJS: 3kB → 4kB (+33%)

Questions:

1. Why were the limits increased so much? Has the bundle actually grown, or is size-limit measuring differently than bundlesize?
2. Can we tighten these limits to prevent future bloat?

Recommendation:
This is good maintenance work, but we should verify the actual current bundle sizes and set limits closer to reality rather than just doubling them.

@erikras - The CI fix is solid, but the size limit changes need review. We want to prevent bundle bloat while being realistic about current sizes.