feat: allow custom edit links

[brc-dd]

fixes: #694

[RainKolwa]

Hi, thank you for this PR. I have a small suggestion:
For GitLab, can we use :repo/edit/:branch/:path(the same to GitHub) as edit link pattern?
This would be more compatible with self-hosted GitLab which is still using older version(e.g. v12).

[kecrily]

For GitLab, can we use :repo/edit/:branch/:path(the same to GitHub) as edit link pattern?

In my PR for the same issue, you can customize the style of the edit links

export default {
  themeConfig: {
    editLink: {
      style: 'github',
      domain: 'gitlab.com',
      repo: 'vuejs/vitepress',
      branch: 'next',
      dir: 'docs',
      text: 'Edit this page on GitHub'
    },
  }
}

More infomation https://github.com/vuejs/vitepress/pull/697/files