Improve shopify theme package to include the release-notes.md file

[karreiro]

WHY are these changes introduced?

The release-notes.md file is excluded when partners run shopify theme package. However, that file is a new requirement to support the Shopify theme update feature.

WHAT is this pull request doing?

This PR changes the Theme::Command::Package class to include the release-notes.md file in the list of zipped assets.

Nitpick, but I've also changed the test to fail if something changes in the list of zipped assets.

How to test your changes?

• Run shopify-dev theme package in a theme directory with a release-notes.md file
• Unzip the file created by the previous step
• Notice that the release-notes.md file is no longer excluded

Post-release steps

None.

Update checklist

• I've added a CHANGELOG entry for this PR (if the change is public-facing)
• I've considered possible cross-platform impacts (Mac, Linux, Windows).
• I've left the version number as is (we'll handle incrementing this when releasing).
• I've included any post-release steps in the section above.