Fix/upload chunk size

[jordan-a-young]

This upgrades tus-js-client to the latest version. This also implements a custom HTTPStack that is a wrapper for the fetch api. This is used to move away from the previes XHR implementation. I added some docs for usage.

[codecov]

Codecov Report

Attention: Patch coverage is 76.84887% with 72 lines in your changes missing coverage. Please review.

Project coverage is 85.06%. Comparing base (f4b8648) to head (c5699c6).
Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
packages/upload-core/src/index.ts	77.82%	51 Missing ⚠️
packages/upload-core/src/http-stack.ts	66.66%	19 Missing ⚠️
packages/api-axios/mocks/handlers.js	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #805   +/-   ##
=======================================
  Coverage   85.05%   85.06%           
=======================================
  Files          85       87    +2     
  Lines        2617     2679   +62     
  Branches      664      674   +10     
=======================================
+ Hits         2226     2279   +53     
- Misses        391      400    +9     

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