Based on this post - https://reasonml.chat/t/ppx-no-need-for-separate-ppx-and-ppx6-with-ocaml-migrate-parsetree-1-6-0/2210 - it seems we can bump the version of OCaml to 4.10 while still preserving backwards compatibility with the current version of BuckleScript.

This would also allow us to use the css-parser dependency from OPAM, instead of duplicating the code (#80)

cc @tatchi for your thoughts