Support multiple "ui" paths

[romannurik]

Figma now supports multiple ui values in manifest.json, like so:

"ui": {
  "main": "main-ui.ts",
  "settings": "settings-ui.ts"
}

It'd be nice for figplug to support that too. Right now it seems to error out with:

/usr/local/bin/figplug: TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received an instance of Object