fix: define window to avoid breakage when using it in the config file

[sarahdayan]

When a user uses window in the config file outside of a function, it will break the export process because the postcss.config.js file imports the config file to get the data under styles. Because window doesn't exist in Node, it throws.

This PR fixes the issue by defining window and setting it to global. We don't care that window doesn't exist in that case because we don't want to get data from it in this context. Styles need to be defined statically for now, because we need them to write the stylesheet.