Parse for line numbers with latest supported ECMA version

This means the supported syntax changes depending on the installed Acorn
version, which may change independently of the AVA version since we do
not pin the dependency.

lib/worker/line-numbers.js

@@ -12,7 +12,7 @@ function parse(file) {
 	const walk = require('acorn-walk');
 
 	const ast = acorn.parse(fs.readFileSync(file, 'utf8'), {
-		ecmaVersion: 11,
+		ecmaVersion: 'latest',
 		locations: true,
 		sourceType: 'module',
 	});