feat: add nx option to define the dist directory of next app

[blaadje]

Allows us to define a param as above :

// apps/showcase/project.json

"browser": {
  "executor": "@nx-plus/nuxt:browser",
  "options": {
    "buildDir": "dist/apps/showcase" // <-- here
  },
  "configurations": {
    "production": {}
  }
},

So we are more flexible on the output.

For my use case it allows me to have something like this