Add editorType property to the manifest.json

[javierarce]

Right now if you create a plugin, the editor complaints of a missing property (editorType):

The default manifest could be something like this:

{
  "api": "1.0.0",
  "editorType": ["figma", "figjam"],
  "name": "My Plugin",
  "main": "plugin.ts",
  "ui": "ui.ts"
}