fix: Update allowed licenses, i.e. GPL2 compatible licenses

[ntwb]

This PR seeks to update the list of allowed GPL2 compatible licenses based on the list currently included in #129

[aduth]

Why should we want WordPress packages to be published with anything other than a GPL license?

I understand from related Slack discussion that there are some exceptions that exist currently, but should we want those exceptions? Or should those packages be relicensed?

Thinking we should want to promote a single license, rather than leaving it as an open question (consistency in expectations for consumers, no uncertainty for developers in choosing license).

[ntwb]

@pento via https://wordpress.slack.com/archives/C5UNMSU4R/p1526992426000277

It’s not just GPL compatible licenses.
w.org allows any OSS license.
It’s only Core that has the strict GPL2 compat requirement.

So that was the basis of my decision to add more licenses to the allowed list.

A follow up discussion suggested we create a @wordpress/licenses package to use as the basic for the list used here and in #129 for the license check package.

An initial pass at the @wordpress/licenses package is in #132

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In relation to the Packages and Gutenberg repos we can restrict the allowed license to GPL-2.0-or-later as is currently done for the respective repos in the npmPackageJsonLintConfig config option in the repo root here

[aduth]

Are not these packages considered "Core" ?

[ntwb]

Issue moved to WordPress/gutenberg #7821 via ZenHub