add codes to warnings

[Rich-Harris]

fixes #474. Realised this isn't actually a breaking change so isn't a prerequisite for v2, but is a good thing to get done nonetheless. Only warnings so far, need to do errors as well.

[codecov-io]

Codecov Report

Merging #1340 into master will increase coverage by 0.02%.
The diff coverage is 79.47%.

@@            Coverage Diff             @@
##           master    #1340      +/-   ##
==========================================
+ Coverage   91.72%   91.74%   +0.02%     
==========================================
  Files         125      125              
  Lines        4505     4509       +4     
  Branches     1430     1431       +1     
==========================================
+ Hits         4132     4137       +5     
  Misses        149      149              
+ Partials      224      223       -1

Impacted Files	Coverage Δ
src/css/Stylesheet.ts	90.62% <ø> (ø)	⬆️
test/parser/index.js	100% <ø> (ø)	⬆️
src/generators/wrapModule.ts	91.02% <ø> (ø)	⬆️
src/validate/js/utils/checkForComputedKeys.ts	33.33% <0%> (ø)	⬆️
src/validate/html/validateHead.ts	60% <0%> (ø)	⬆️
src/validate/js/propValidators/immutable.ts	0% <0%> (ø)	⬆️
src/parse/state/mustache.ts	89.41% <0%> (ø)	⬆️
src/validate/js/propValidators/actions.ts	50% <0%> (ø)	⬆️
src/validate/js/propValidators/setup.ts	50% <0%> (ø)	⬆️
src/validate/html/index.ts	97.87% <100%> (ø)	⬆️
... and 29 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1c4b166...35f4a1f. Read the comment docs.

[Rich-Harris]

This was fantastically tedious. I hope it proves useful to someone someday.