feat: add proper ESM support for modern bundlers

[pumpkinzomb]

Overview

This PR improves the fonteditor-core library to work seamlessly with modern ESM-compatible environments such as Vite, Next.js, Webpack 5, and others. The current codebase mixes CommonJS and ESM syntax, causing import errors when used in ESM environments.

Changes

• Implemented dual package exports pattern in package.json (supporting both CommonJS and ESM)
• Added an ESM-specific entry point file (main.mjs)
• Modified build script to include ESM module files in the build output
• Improved TypeScript declaration file path references
• Added documentation for ESM usage with a markdown guide

Testing

• All existing tests pass
• Verified working correctly in ESM environments (Vite, Next.js)

With these changes, users can import fonteditor-core directly as an ESM module in modern bundlers without additional configuration.