Add scalaCompilerPlugins to Mill project export

[carlosedp]

This PR adds the export of scala compiler plugins to Mill projects.

A file like:

//> using scala "2.13.10"
//> using lib "edu.berkeley.cs::chisel3::3.5.5"
//> using plugin "edu.berkeley.cs:::chisel3-plugin::3.5.5"

import chisel3._
import circt.stage.ChiselStage
...

Generates a build.sc:

import mill._
import mill.scalalib._

object memmask extends ScalaModule {
  def scalaVersion = "2.13.10"
  
  def ivyDeps = super.ivyDeps() ++ Seq(
  ivy"edu.berkeley.cs::chisel3::3.5.5",
)

  def scalacPluginIvyDeps = super.scalacPluginIvyDeps() ++ Seq(
  ivy"edu.berkeley.cs:::chisel3-plugin::3.5.5"
)
...

[lwronski]

LGTM, thanks for your contribution.