Separate typings for node and browser

[frenzzy]

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[codecov[bot]]

Codecov Report

Merging #10 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #10   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines          91     91           
  Branches       18     18           
=====================================
  Hits           91     91

Impacted Files	Coverage Δ
src/node.js	100% <100%> (ø)	:arrow_up:
src/browser.js	100% <100%> (ø)	:arrow_up:

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