Refactor: CssWriter

[lukeed]

Initially just added types from #72; but then decided to extract an emit function to minimize potential misuse within the css function. AKA, by exposing the entire PluginContext within the css function, one could easily fumble around and break their Rollup build. By just exposing this.emit, we still offer flexibility of emitting additional assets within the css() callback, if/when needed.

[benmccann]

the other option here that might be easier is to just make some of the fields like bundle private. I don't think we really need to expose bundle or emit actually

[lukeed]

Having this.emit could be nice since it allows the dev to have a final window to emit extra assets into the bundle; especially since we expose filename too now.

[pngwn]

Yep, this makes sense. Looks good to go. 👍