Better way to define custom builtin cells (stdlib)

[asg017]

window.DATAFLOW_STDLIB works well with run/compile, but it's awkward, constant/dependency is confusing, and you can only be dependent on 1 stdlib cell. This should be done in a newer way.

Maybe:

export function d3() {
  return {
    depend: ["require"],
    define: require => require("d3@6")
  }
}

• I like using ES modules instead of window stuff here
• depend is awkward
• define is probably as best as we will get