feat: add types

Add types for postcss-js, so users don't need to install @types/postcss-js separately.

In general, the idea looks good for me. Make it Ready from review when it will be ready.

I have problem when overriding PostCSS's Processor#process. When this problem is resolved, i can make it Ready to review. Maybe you can help me?

I have problem when overriding PostCSS's Processor#process. When this problem is resolved, i can make it Ready to review. Maybe you can help me?

Can you describe the problem more?