Fix/ssr css variables

[m0ksem]

closes #3046

Added a build plugin which converts v-bind(variable) in <style> to :style="".

Basically, creates a css variable like --va-[index]-[kebab-name] and pass it to style using v-bind and replaces vue v-bind in css with generated css variable. This way we ignore vue useCSSVars which is not available in cjs build. We also ensure that passed value to css variable is going to be String, which makes it work in Nuxt.

[aluarius]

Logic is pretty hard for understanding, please comment it as much as possible.

[aluarius]

Not sure if related to this pr, but if you're using light theme of the nuxt-docs and then go to http://localhost:3000/en/styles/colors, it automatically switches to the dark one.

That's pretty weird.