feat(target_typescript): add support for additionalProperties

[mashizora]

Add support for additionalProperties in typescript output target.

Example

JSON Typedef:

{
    "properties": {
        "name": { "type": "string" },
        "isAdmin": { "type": "boolean" }
    },
    "additionalProperties": true
}

Generated TypeScript code:

export interface Root {
  isAdmin: boolean;
  name: string;
  [x: string]: unknown;
}