We need a way for AVA to know whether to load a particular test file (given its extension) as commonjs or module, similar to the "type" field in package.json.

It must not be possible to change how cjs and mjs extensions are loaded. And I have a strong suspicion (but I have not confirmed this) that it's impossible to override the "type" field (which applies to js extensions).

Once configured as module, we then need to import() these test files. This is blocked by #2344.