Merge options: css + emitCss

[lukeed]

While working on #145 (and previously too, tbh) I feel like it might make more sense to consolidate emitCss and css into one option.

It could be a single extractCss option.

• false – sets compilerOptions.css: false, resulting in inlined CSS
• true – similar to emitCss: true¹
• function – sets compilerOptions.css: false, and uses custom CssWriter to produce a single CSS output.

Prior Art

This would follow what rollup-plugin-postcss has been doing, and feels very intuitive. We can follow it more closely, and allow boolean | string values only, where string uses a CssWriter function internally (the 90% default anyway).

¹ Our emitCss should be emitting CSS files individually only. Right now, they're given importd lines but are still concatenated into the single file that CssWriter produces anyway (bug).