Merge island_settings & island_configuration in profiles

Problem/Motivation

Display builder profile config entity have both island_settings & island_configuration:

island_settings:
  view:
    builder:
      enable: true
      weight: '-10'
      options: main
    layers:
      enable: true
      weight: '-9'
      options: main
    preview:
      enable: true
      weight: '-8'
      options: main
  button:
    history:
      enable: true
      weight: '-8'
    state:
      enable: true
      weight: '-7'
  ...
island_configuration:
  history:
    display_clear_button: 1

This is confusing, not enough defined (type: ignore), and may introduce bugs because:

• island_settings follows to closely the form structure
• we need to maintain 2 lists with the same keys

Proposed resolution

Can we have a merged flat structure managed by something like Drupal\Component\Plugin\LazyPluginCollection

Proposal:

islands:
  builder:
    enable: true
    weight: -10
    region: main
  layers:
    enable: true
    weight: -9
    region: main
  preview:
    enable: true
    weight: -8
    options: main
  history:
    enable: true
    weight: -8
    settings:
      display_clear_button: 1
  state:
    enable: true
    weight: -7
  ...

This is also the opportunity to allow configuration of islands before first config save and to remove:

Island configuration will be available only after saving this form.