Support ava.config.mjs files

[novemberborn]

AVA recognizes ava.config.mjs files but refuses to load them. Let's fix this!

See the code here:

ava/lib/load-config.js

Line 109 in 7f99aef

throw new Error(`AVA cannot yet load ${fileForErrorMessage} files`);

Of course import() is only available on Node.js 13. This should fail gracefully on Node.js 10 and 12.

import() is asynchronous. The loadConfig() method should still return synchronously when loading other files.

The documentation needs to be updated to explain when .mjs config files make sense and when users should use .cjs or .js files. Mostly this is to do with ESLint compatibility (since rules must currently be synchronous).

[arlac77]

Why not switching to async loadConfig() ?

[novemberborn]

This is to do with ESLint compatibility (since rules must currently be synchronous).