Add browser support

[ai]

PostCSS is often packaged to JS bundle. We need index.browser.js version. It is OK just to return string there (but you will need to convert undefined and numbers to strings to be compatible with types).

Here a way to use them in package.json: https://github.com/ai/nanocolors/blob/main/package.json#L20-L23 https://github.com/ai/nanocolors/blob/main/package.json#L28

[alexeyraspopov]

picocolors@0.1.0 is now published and supports browser field
https://github.com/alexeyraspopov/picocolors/releases/tag/v0.1.0