feat: adding type definitions for basic clay interfaces

[flamerohr]

Hello, this is an attempt to add type definitions to Clay, focused mostly on the config json that defines how to build the form.

The way I've found to use the types is to create a config.js file with content:

/**
 * @type {import('@rebble/clay').Config}
 */
module.exports = [
  {
    type: "heading",
    defaultValue: "My heading",
  }
];