Include assets from "assets/" directory (#126)

[mwistrand]

Copy (without hashing) all files in the assets directory to the output/{mode}/assets directory.

This commit has already been merged to next, but now requires a separate PR to master.

[codecov]

Codecov Report

Merging #130 into master will decrease coverage by 0.76%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
- Coverage   60.53%   59.76%   -0.77%     
==========================================
  Files           8        8              
  Lines         413      425      +12     
  Branches       92       94       +2     
==========================================
+ Hits          250      254       +4     
- Misses        163      171       +8

Impacted Files	Coverage Δ
src/dist.config.ts	36.17% <33.33%> (-0.42%)	⬇️
src/dev.config.ts	26.53% <33.33%> (+0.94%)	⬆️

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