build: make copy-to-clipboard an explicit dependency

[hydrosquall]

Motivation

When using this plugin inside a yarn 2 project, an error message is thrown because copy-to-clipboard was referenced even though it's not part of the package's dependencies.

addon-knobs/src/components/Panel.tsx

Line 6 in 1270182

import copy from 'copy-to-clipboard';

Project owners can manually install copy-to-clipboard themselves, but including it within the library should saves people from an extra error message.

Notes

• This should not conflict with the version of copy-to-clipboard currently in the mainline storybook branch. https://github.com/storybookjs/storybook/blob/3c076a333a122feeb07af520c28195358bab68ba/lib/ui/package.json#L54 . I think this import is why the library works indirectly (the issue just wasn't flagged until using yarn 2, since it's stricter about imports).