Publish nightly releases as part of the CI, so Ruffle can be easily
integrated into a website using a CDN (such as jsDelivr). It would
also be easier for websites to keep up-to-date, as NPM auto-updates
the `latest` tag to refer to the latest release.
A dedicated `package.json` is used for the published NPM package,
because `ruffle-selfhosted` depends on the private `ruffle-core`
package, which shouldn't be published.
Also, the `version` field must monotonically increase. So Webpack
auto-fills it to be `0.1.0-nightly.YYYY.MM.DD`. This format satisfies
a couple of needs:
* Newer nightly releases should take precedence over older ones.
* Stable releases (e.g. `1.0.0`) should take precedence over nightly
ones.
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