implement "pnpm clean" for src/packages/assets

[williamstein]

I don't understand exactly what gets built in this package. In all other packages there is pnpm clean which deletes, e.g., the dist subdirectory, which contains all built code.

I frequently do pnpm clean && pnpm build-dev && pnpm test to do a quick 3-4 minute test that things are working. This is a much more useful test if pnpm clean fully works.

[williamstein]

I also do pnpm clean && pnpm build-dev when switching between different git branches, and again, it's best if pnpm clean actually fully works. Hence motivation for this.