Modular Scripts should come with postcss installed if using the postcss rollup plugin

[Cang]

We instruct rollup to use rollup-plugin-postcss to build libs with .css files but it requires postcss >= 8 to be installed as a dependency, as outlined in this error message received when trying to build a lib:

Error: PostCSS plugin postcss-noop-plugin requires PostCSS 8.
Migration guide for end-users:
https://github.com/postcss/postcss/wiki/PostCSS-8-for-end-users

https://github.com/postcss/postcss/wiki/PostCSS-8-for-end-users#rollup

[Luke Sheard]

Closed in #435