Update configurations / dependencies to the latest versions.

[mrvicadai]

Changelog

• Fix bsconfig.json warning by specifying refmt: 3
• Use yarn as default package manager.
• Update devDependencies to latest versions.
• Default to use Main.re instead of Main.ml.
• Various formatting.

[tcoopman]

I'm fine with updating the config, but I'm not sure about having a default for reason. Can you provide both ml and re files like in https://github.com/OvermindDL1/bucklescript-tea?

[mrvicadai]

@tcoopman thanks for reviewing! Just addressed the comment by reverting to Main.ml in https://github.com/tcoopman/bucklescript-tea-starter-kit/pull/3/commits/a2ce86513366d132e574bf63479dd09f9b3b0698