Add TSDoc types

[mxdvl]

Add TSDoc style types in the index.js, and add a script to generate the declaration file automatically, so these types are available to consumers.

npx -p typescript tsc index.js --declaration --allowJs --emitDeclarationOnly --outDir .

Fixes #6

[mxdvl]

Hey @lukeed, anything I can do to help push this over the line?

[mxdvl]

Closing as there seems to be little appetite for this 😊