feat: add defineAction method for defining custom templates

[gyoshev]

With this PR, the JS SDK allows defining custom message templates to be consumed through a website.

Usage:

    // Define the action name and fields that will show up in the Leanplum dashboard
    Leanplum.defineAction({
      // name shown in the template picker
      name: 'Custom Popup',
      args: [
        // properties in message editor
        { name: 'Title',   type: ActionParameterType.String, value: 'Title placeholder' },
        { name: 'Content', type: ActionParameterType.Text,   value: 'Content placeholder' },
        { name: 'Image',   type: ActionParameterType.File,   value: '' },

        { name: 'Layout',  type: ActionParameterType.Group,  value: [
          { name: 'Width',   type: ActionParameterType.Number, value: 800 },
          { name: 'Height',  type: ActionParameterType.Number, value: 600 }
        ] },

        { name: 'Dismiss action', type: ActionParameterType.Action, value: '' }
      ]
    });

    // Render the action by processing the "showMessage" event
    Leanplum.on('showMessage', (args: RenderOptions) => {
      const { message, context } = args;

      if (message.__name__ === 'Custom Popup') {
        // render popup and use context
      }
   })