feat: implement vite `vite:post-css` support

[e111077]

Vite does a weird thing on its production build where it will make it export a literal rather than a variable e.g.

// normal postcss output
var css_hash = "...";

export default css_hash;

// vite output
export default "...";

This adds support for the latter

fixes e111077/vite-lit-element-ts-sass#3

[VitaliiK91]

This resolves the issue with vite production build!