Make execa compatible with Node.js 13.0.0-pre

Use `error.code` instead of `error.errno` when expecting a string error
code. `errno` is always numeric in Node.js 13.0.0-pre.

Node.js 13.0.0 comes out later this month (October 2019) so implementing
and publishing this change will hopefully smooth the upgrade path for
execa users.

index.js

@@ -190,7 +190,7 @@ module.exports.sync = (file, args, options) => {
 			code: result.status,
 			command,
 			parsed,
-			timedOut: result.error && result.error.errno === 'ETIMEDOUT',
+			timedOut: result.error && result.error.code === 'ETIMEDOUT',
 			isCanceled: false,
 			killed: result.signal !== null
 		});