Move polyfill resolution from webpack to package.json

[fgnass]

Summary

This PR simplifies the setup for users consuming decap-cms-app directly by addressing Node.js built-in dependencies in a different way.

Problem:

1. Many decap packages depend on Node.js built-ins (particularly path)
2. Currently, these are only polyfilled via webpack configuration in the pre-built bundle
3. Users consuming decap-cms-app directly must manually configure their build tools to provide these polyfills

Solution:

1. Leverage the standard browser field in package.json to specify browser-compatible alternatives
   • This is widely supported by modern build tools (webpack, Vite, Rollup, etc.)
   • Eliminates need for manual polyfill configuration
2. Reduce polyfill requirements by:
   • Using @iarna/toml/parse-string instead of full @iarna/toml to eliminate stream dependency
   • Updating minimatch to a more recent version which removes Node.js built-in dependencies

These changes make the library more portable and easier to integrate across different build environments.

Test plan

• Build succeeds ✅
• tests:all pass ✅
• demo runs ✅

Checklist

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