Build improvements

[fregante]

Following #44

• Don't minify the CSS
• include webext-base-css via @import 'webext-base-css';
• Flat output How can I get all the output files in a single, flat directory? parcel-bundler/parcel#5858
• Somehow extract webext-options-sync to its own "vendor" entry so it's not bundled 3 times separately [Web Extension Transformer] doesn't support code splitting parcel-bundler/parcel#5859
• Automatically include the browser global

The last one is feasible via:

https://github.com/pixiebrix/webext-messenger/blob/35961df965d1a75da941187ddcb67acdaeb82aec/test/demo-extension/manifest.json#L7

And

https://github.com/pixiebrix/webext-messenger/blob/35961df965d1a75da941187ddcb67acdaeb82aec/.parcelrc#L4-L9

[fregante]

Some Parcel config can be copied from pixiebrix/webext-messenger@2a4ab7d#diff-7ae45ad102eab3b6d7e7896acd08c427a9b25b346470d7bc6507b6481575d519