migrate withStorybook to typescript

[tlow92]

Proposal to fix https://github.com/storybookjs/react-native/issues/630

[!NOTE] Instead of manually defining the d.ts file I converted the current implementation to typescript and let tsup emit the .js and .d.ts file. This should hopefully be more future-proof.

In general it works, but I had to add a custom tsup cli cmd to the package.json (reason is explained below).

• Currently withStorybook is imported like this require('@storybook/react-native/metro/withStorybook'); I think ideally withStorybook would live in src like all other files and be transpiled to dist, but that would be a breaking change.
• I tried to emit the files to dist and put barrel files in metro folder so that it is not a breaking change, but that created problems with tsup because apparently it can not handle barrel .d.ts files and always emits an empty .d.ts.
• The other approach is what I did now: Using a separate tsup cmd to specify a different outDir (metro folder)

[dannyhw]

thanks for getting this underway! I think we got a good solution here. I'm going to merge and start working toward a 8.4 pre-release with multiple changes in it

[trajano]

I looked at the code the hack I did with export = withStorybook is not needed if you converted to typescript instead just export function withStorybook in an earlier line.

The hack was needed because of the d.ts nature which no longer applies.