[Script request] Add posts cover Front-Matter for themes

[remi-guan]

Theme Branch:

• Aurora：cover
• Fluid：index_img 95961a4
• Stun：top_image
• Icarus：cover
• Keep：V/A
• Kaze： banner_img
• Inside：thumbnail
• Stellar: cover
• Minidyne: V/A
• Landscape: V/A

Describe the solution you'd like
Change cover: <link> in the posts front-matter to the themes required field, for example, theme Fluid uses index_img, so we need this script.

// scripts/meta-space-post-filter.js
hexo.extend.filter.register('before_post_render', function (data) {
  data.cover && (data.index_img = data.cover);
  return data;
});

Additional context:

If you're resolving one of the pending requests, please comment and link to this issue in your PR, so I can mark it as resolved.