[preset-storysource] Not published to NPM about presets

[jerriclynsjohn]

@shilman is this part of the addon package itself, If that's the case then we should update the readme as it mentions @storybook/preset-storysource , also same error because the package doesn't exist.

[shilman]

We're going to move this into the addon--it doesn't need its own package. If you'd like to do that, I'd def review your PR!

[shilman]

@jerriclynsjohn See my PR: #43

[doublejosh]

Looks like the type definitions are also not published...
An unexpected error occurred: "https://registry.npmjs.org/@types%2fstorybook__addon-storysource: Not found"