chore(types): convert to TypeScript

[benjie]

This is currently the result of running this against the codebase:
bcherny/flow-to-typescript#12

TODO:

• Automated conversion on all Flow files
• Run prettier on all converted files to make them match the Flow as much as possible
• Delete the .js files, hopefully git recognises the .js -> .ts as basically a rename?
• Review differences, fix where appropriate
• Remove Flow from everywhere
• Add relevant TypeScript compilation stuff
• Ensure all the tests pass
• Have a hard-earned beer 🍺

Convert script:

git checkout 686ee4e40a9f301d91cda7d42e393582fae94026 .
for I in packages/*/src/*.js packages/*/src/**/*.js; do echo $I; ./flow-to-typescript/dist/src/cli.js -i "$I" -o "${I/.js/.ts}" && rm "$I"; done
yarn prettier:fix
git add packages

[benjie]

Closing in favour of #573