Centralize the `update-contributors` utility

[jginsburgn]

Updating the contributors in the corresponding field of the package.json file is one of the tasks that CD undertakes in every release of at least the Karma core and the Karma Chrome Launcher plugin. However, the implementation for that tool is duplicated in at least each of the aforementioned repositories; code duplication is a bad practice and the proposal would be to centralize the implementation of the tool under the Karma Runner org and release it as an npm module under the @karma scope (e.g. @karma/update-contributors). An alternative is to use an existing npm module that does this; a list of potential options follows, but they have been stale for a while.

Some existing tools for this task already exist:

• https://github.com/flo-sch/semantic-release-contributors
• https://github.com/doowb/update-contributors