Feat/new game dynamic map list

[flyingmongoose]

Implemented dynamic map list on the New Game UI.

---

I have created a wiki page on my own fork to provide details: 🗺️ Community Maps

But here's the short version

1. Bundled manifest (res://Assets/World/maps_manifest.json)
2. Bundled directory scan (res://Assets/World/World*.tscn)
3. User manifest (user://maps/maps_manifest.json)
4. User directory scan (user://maps/World*.tscn)

I have also included an interpretation of a Metadata string on the root node of a map.
If the map manifest file does not include the map, or map name, it will load from a metadata string called MapName.
This must be on the root node of the scene for the map.

---

Maps Manifest File (maps_manifest.json) Format:

{
  "maps": [
    {
      "name": "WorldDev2D",
      "path": "res://Assets/World/WorldDev2D.tscn"
    },
    {
      "name": "WorldDev2D_perf",
      "path": "res://Assets/World/WorldDev2D_perf.tscn"
    },
    {
      "name": "WorldDev2D_simple",
      "path": "res://Assets/World/WorldDev2D_simple.tscn"
    }
  ]
}

[LinuxDonald]

@pingwindev could you please review it?