allow resolve config file to use loaders

[panva]

The resolveConfigFile function currently restricts the resolved files to .js, .cjs, or .mjs files.

ava/lib/load-config.js

Lines 40 to 42 in 7007f13

	if (!configFile.endsWith('.js') && !configFile.endsWith('.cjs') && !configFile.endsWith('.mjs')) {
	throw new Error('Config files must have .js, .cjs or .mjs extensions');
	}

What I'm hoping to use, without having to patch ava myself, is a .ts file and use a loader pushed on to node through an environment variable like so

export NODE_OPTIONS='--experimental-loader=@esbuild-kit/esm-loader'

The config file in question performs setup and having it typed helps a lot.