Make type definitions for DefaultTheme available to end-user

[nonameolsson]

It is not possible to use variables defined in the theme in a styled-component right now.

1. New script that makes type definitions available in dist.

• Create a script that generates a global main.d.ts file that contains references to the main.d.ts-file in each component.
• Copy all src/**/*/main.d.ts to dist.

2. Usage of the script

• Run the script as part of the build process
• Don't include /src in the built package published to NPM.
• Add instructions on how to import type definitions in project.

[stale]

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