Add Figma Slots support

[amotarao]

Summary

Figma has introduced a Slots feature, but the REST API spec does not yet define the related types.
This causes issues for API clients that rely on the spec for type generation.

Missing definitions

1. Node type: SLOT

A new node type SLOT should be added to the node types list.

2. ComponentProperty — type: "SLOT"

{
  "componentProperties": {
    "Slot#22:0": {
      "type": "SLOT",
      "value": {
        "guid": {
          "sessionID": 505,
          "localID": 573
        }
      },
      "preferredValues": [
        {
          "type": "COMPONENT",
          "key": "b62c9bdf85ab22b666b3b73b360349328694168e"
        }
      ]
    }
  }
}

3. ComponentPropertyDefinition — type: "SLOT"

{
  "componentPropertyDefinitions": {
    "Slot#22:0": {
      "type": "SLOT",
      "defaultValue": {
        "guid": {
          "sessionID": -1,
          "localID": -1
        }
      },
      "preferredValues": [
        {
          "type": "COMPONENT",
          "key": "b62c9bdf85ab22b666b3b73b360349328694168e"
        }
      ]
    }
  }
}

Impact

Without these definitions, clients using the spec (e.g. for type generation) encounter unknown
types at runtime.

Notes

The full structure of Slots may not be fully documented here.
Additional properties or node interactions may exist that aren't captured in these examples.
I would appreciate any feedback or additional information from the Figma team regarding the full implementation.