[now-static-build] Add caching headers for Stencil

[simonhaenisch]

/cc @adamdbradley @leo

[adamdbradley]

This looks great! 👍

[codecov-io]

Codecov Report

Merging #3493 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3493      +/-   ##
==========================================
- Coverage   14.59%   14.56%   -0.04%     
==========================================
  Files         267      267              
  Lines       10779    10774       -5     
  Branches     1483     1484       +1     
==========================================
- Hits         1573     1569       -4     
+ Misses       9039     9038       -1     
  Partials      167      167

Impacted Files	Coverage Δ
src/util/metrics.ts	61.53% <0%> (-6.65%)	⬇️
src/util/prefer-v2-deployment.ts	55.1% <0%> (-4.9%)	⬇️
src/util/projects/get-project-by-id-or-name.ts	19.04% <0%> (-4.49%)	⬇️
src/util/dev/server.ts	59.66% <0%> (+0.05%)	⬆️
src/util/dev/builder.ts	65.88% <0%> (+0.59%)	⬆️
src/util/get-update-command.ts	10% <0%> (+0.62%)	⬆️
src/util/dev/yarn-installer.ts	83.33% <0%> (+3.64%)	⬆️

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[simonhaenisch]

I've done the requested changes... would someone mind having a look again, please? Thanks!