implement preload

[Rich-Harris]

Closes #1015

[codecov-io]

Codecov Report

Merging #1021 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1021      +/-   ##
=========================================
+ Coverage    92.1%   92.1%   +<.01%     
=========================================
  Files         115     115              
  Lines        4304    4308       +4     
  Branches     1373    1376       +3     
=========================================
+ Hits         3964    3968       +4     
  Misses        147     147              
  Partials      193     193

Impacted Files	Coverage Δ
src/validate/js/propValidators/index.ts	100% <ø> (ø)	⬆️
src/generators/server-side-rendering/index.ts	96.96% <100%> (+0.04%)	⬆️
src/generators/Generator.ts	94.44% <100%> (+0.02%)	⬆️
src/generators/dom/index.ts	95.4% <100%> (+0.02%)	⬆️

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