Fix ESM and CJS module support

[hv068]

What was wrong:

• Missing exports field - Modern Node.js and bundlers couldn't properly resolve the package
• No type field - Caused ambiguity about whether this is an ESM or CJS package
• tsup in main dependencies - Build tool was being installed in production
• Inconsistent file extensions - Mixed .mjs and .js naming

What was fixed:

• Added exports field for proper module resolution
• Set "type": "module" to clarify this is an ESM package
• Moved tsup to devDependencies
• Standardized output to .js (ESM) and .cjs (CommonJS)

Result:
Package now properly supports both import and require() with modern best practices and without issues cause earlier (constructor in types not detected, forcing client to use CommonJS)