Preparation for browser support

[ksaaskil]

First steps to get unmock running in browser and React Native.

• Added unmock-browser package
• Added separate tsconfig.json for the Node.js and browser builds in unmock package so one can tweak their TypeScript configs independently
• Replaced jsonschema with @meeshkanml/jsonschema, a version that should not depend on any Node.js types (see the repo)
• unmock-browser package is quite a simple wrapper around unmock-fetch: when user calls unmock.on(), the package replaces window.fetch with a fake fetch using unmock-core to serve back responses
• Browser package does not contain any tests yet as I'm not sure how to best run stuff in browser (Karma?) and it does not seem trivial to compile the Lerna project to run in browser

NOTE: unmock-browser does not yet work in browser, at least json-schema-ref-parser used by unmock-core depends on Node.js stuff

^^But I'd rather work on fixing that once the packages are in-place.

[codecov-io]

Codecov Report

Merging #348 into dev will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##              dev     #348   +/-   ##
=======================================
  Coverage   80.95%   80.95%           
=======================================
  Files          52       52           
  Lines        2347     2347           
  Branches      577      577           
=======================================
  Hits         1900     1900           
  Misses        447      447

Impacted Files	Coverage Δ
packages/openapi-refinements/src/index.ts	73.73% <100%> (ø)	:arrow_up:
packages/unmock-core/src/generator.ts	87.64% <100%> (ø)	:arrow_up:

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