Factorize/Improve post type definitions

[kuuak]

Post type definition need currently to be defined in multiple place in order for CPT to work properly.
Refactor to improve DX or at least fix the documentation.

eg.

• superstack/src/lib/get-preview-node.ts

  Line 4 in b66ec81

  const POST_TYPES = ['Page', 'Post'];

• superstack/src/lib/get-all-uris.ts

  Line 5 in b66ec81

  const POST_TYPES = ['pages', 'posts'];