Rename javy-core to javy-plugin

[jeffcharles]

Description of the change

Renaming javy-core to javy-plugin.

I've updated the name in our documentation but I will go through the documentation more holistically in the future to ensure it's up-to-date as opposed to doing that as part of this PR. I have also tested the release GitHub Action to determine that it is working correctly with this change.

Why am I making this change?

Part of #768. The javy-core crate is the default plugin for Javy so a name like javy-plugin better reflects that.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-core do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.

[saulecabrera]

Looks good to me -- the test_core step in CI seems stuck, I think it's due to the name of the jobs?

[jeffcharles]

It's because the test_core check is marked as mandatory but the name of the workflow has been changed to test_plugin. So I need to remove test_core as a mandatory check before I can merge and then I can mark test_plugin as mandatory.

[jeffcharles]

Branch protection rules have been updated