feat: add TypeScript definitions

[malept]

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• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Also moves the API docs from a manually-curated Markdown file to the TypeScript definition, which also allows us to generate GitHub Pages (like Electron Packager's).

[codecov]

Codecov Report

Merging #72 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #72   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          252       252           
=========================================
  Hits           252       252           

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