Publish to opam

[davesnx]

At @ahrefs we use it with Melange, but currently documentation and libs are designed for ReScript.

To make Melange a first-class citizen:

• [x] Support Melange 3 (with reason-react 0.14.0) https://github.com/davesnx/styled-ppx/pull/423
• [ ] Have proper package names
  • [x] Rename styled-ppx.lib -> styled-ppx.ppx or just styled-ppx
  • [ ] styled-ppx.emotion + styled-ppx.css should be called "styled-ppx.runtime"
  • [x] styled-ppx.emotion-hash -> styled-ppx.murmur2
• [ ] Documentation page (explain how to install and consume)
• [x] Rewrite README.md
• [x] Have a language switcher (ReScript/Reason/OCaml) for code-snippets
• [x] Remove pcre from server-reason-react.js (fix https://github.com/ml-in-barcelona/server-reason-react/issues/45)
• [x] Publish to opam

This issue should close https://github.com/davesnx/styled-ppx/pull/409