Obsolete prepare script

[gchoqueux]

• This script prepare.mjs is clearly outdated (some dev dependencies now require node > 18.0) and we could simply specify the requirement for both node and npm directly in the package.json with

  "engines" : { 
    "node" : ">=18.0.0"
  }

• Since we now have an ESM distribution, I think we could remove the ThreeExtended script and directly imports from three/addons/*.

Originally posted by @Desplandis in #2300 (comment)