Fix @smooth-ui/core-em TypeScript definitions

[elliottsj]

Summary

Adjust the build step so core-em and core-sc have their own independent type definitions, and copy the shared definitions to a 'shared/index.d.ts' module.

Fixes #106

Test plan

Check out my branch, then yarn run build the repo. Then run:

npm install ./smooth-ui/packages/core-em @emotion/core @emotion/styled emotion-theming typescript @types/react
echo "import { Button } from '@smooth-ui/core-em';" > index.ts
./node_modules/.bin/tsc index.ts

The tsc command should exit with status 0.

I have tested this with my own application which uses emotion + TypeScript, but I haven't tested with a styled-components project yet. I'd also like to add a tsc build/test step to smooth-ui to prevent future regressions. Let me know if I should include it as part of this PR or a follow-up PR, and any other suggestions!

[gregberge]

Hello @elliottsj, thanks for your work! I think adding a build step to this PR is a good idea!

[elliottsj]

Okay, I've added a tsc:check npm script which will verify both index.d.ts files as part of the CI build.