Refactor and deploy some tricks so that ava/plugin is usable in a CJS environment, even if the rest of AVA's internals become ESM.

* Load yargs as CJS due to strange compatibility issues
* Keep line number parsing code as CJS because it attempts to lazy-load dependencies
* Shared workers are always loaded through a dynamic import
* Shared worker filenames must now be file:// URLs, but you can provide a URL instance
* Test file paths within shared workers are now file:// URLs
* Error serialization and code excerpts now support ESM
* Title prefixes in the reporters now correctly skip all test file extensions, not just `js`