Remove yarn scripts, yarn styles, yarn copy tasks

[anselmbradford]

There's a bug in the yarn scripts task where it needs a loader for SCSS. Instead of fixing that, I'm proposing ripping out the yarn scripts, yarn styles and yarn copy tasks and just having a single yarn build (and accompanying yarn watch) task. Build doesn't take that long, and that simplifies our maintenance of these scripts.

Removals

• Remove yarn scripts, yarn styles, yarn copy tasks

How to test this PR

1. yarn build and yarn watch should run without error.

[wpears]

Suuuuuuuure. The bug is expected, since esbuild doesn't support scss. The only way the build works in the design system is by including the postcss sass plugin: https://github.com/cfpb/design-system/blob/main/esbuild/packages/scripts.js. But I agree this is fine in general. I do sometimes run these scripts individually, but watching is usually the better move.