MVP-first version

Through hours of trials and errors, Finally cope with basePath problem in two conditions: CustomDomain and GithubPages.
Now we can visit site, via langshift.jackcheng.chat

// the right setting
const basePath = isCustomDomain ? '' : (isGithubPages ? '/langshift' : '');
// previous setting have some problem in visiting custom domain site.
const basePath = isCustomDomain ? '' : '/langshift';
const basePath = isCustomDomain ? '' : (isGithubPages ? '/langshift' : '/langshift');

Full Changelog: https://github.com/anzchy/langshift/commits/0.5.0