specify css exports in package.json

[TheSharpieOne]

Currently only the JS is specified in the exports within package.json. When exports field is present, it means anything not explicitly listed inside is not exposed. This means the css is not exposed. This results in this error message when using vite:

[commonjs--resolver] Missing "./dist/handy-scroll.css" specifier in "handy-scroll" package
error during build:
Error: Missing "./dist/handy-scroll.css" specifier in "handy-scroll" package

Per vitejs/vite#2657 (reply in thread), a separate export can be added to expose the css.

How I tested: I updated the exports package.json within my node_modules/handy-scroll where I was having this issue and this change resolved the issue.