Simplify CI workflows

[jeffcharles]

Description of the change

This is an attempt at simplifying the CI workflows. Instead of using two jobs for CI, it switches to one. We used two because we used to compile the plugin with MacOS but do the CLI integration testing with Linux. But now we compile the plugin with Linux so we can reuse the same job. I've also moved the WPT tests because they can just re-use the same artifact we've built in the CI pipeline to run the WPT test suite so we don't need a separate workflow. We used to need a separate one because the WPT test suite required Javy to be built with an experimental_event_loop feature enabled but that's not the case any more. I've also inlined the shared CI setup since there's only one user of it now.

Why am I making this change?

Less workflows to manage and reducing the jobs from two to one should speed up CI a bit.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-plugin 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.