Allow configuring postcss and sass/stylus/css modules via moduleServer options

[calebeby]

Currently we have a way to configure or disable esbuild via moduleServer options. We should have the same kind of thing for the CSS processing.

It needs to apply the options in the two places where the CSS plugin is used:

• In the plugins array in the module server (i.e. if you do import "./asdf.css" in JS)
• In the CSS middleware (i.e. if you do @import "./asdf.css" in CSS)

Most of the options we will need are the same as: https://github.com/egoist/rollup-plugin-postcss (we do not need all of their options, just a subset).

Here are the things that I think should be possible with the added options (we should have tests for these):

• Importing CSS modules
• Enabling postcss plugins
• Configuring sass/scss options

[calebeby]

I realized that rollup-plugin-postcss accounts for over half of pleasantest's dependency graph, and most of the time it isn't even used, except for loading plain CSS without transformations.

graph of Pleasantest's dependencies
graph of rollup-plugin-postcss's dependencies

I would rather make postcss an optional dependency that people can add, rather than including it by default. Because of that, we cannot depend on rollup-plugin-postcss.

I'm marking this as blocked till we find another CSS solution

[calebeby]

Closing this to be a part of #240