feat: add subscriptions

[braposo]

This is the code that @okonet has in his fork to implement subscriptions to the Figma API.

I'm just leaving this here for reference and let's discuss it and improve when possible.

This shouldn't be merged for now.

[codecov-io]

Codecov Report

Merging #13 into master will decrease coverage by 3.4%. The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
- Coverage   73.33%   69.92%   -3.41%     
==========================================
  Files          17       17              
  Lines         120      133      +13     
  Branches        8       10       +2     
==========================================
+ Hits           88       93       +5     
- Misses         25       32       +7     
- Partials        7        8       +1

Impacted Files	Coverage Δ
src/types/file.js	47.61% <44.44%> (-14.89%)	:arrow_down:

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