feat: new plugins definition syntax

[ubermanu]

plugins should return a more complex interface

export interface Plugin {
  /** The name of the plugin. */
  name: string

  /** The build function that is called when the plugin is executed. */
  build: (context: PluginContext) => MaybePromise<void>

  watcherConfig?: {
    /** The files to reload if the right counterpart matches a changed file. If the key is '*', the whole page is reloaded. */
    reload?: {
      [file: string]: string | RegExp | Array<string | RegExp>
    }
  }
}

• watcher should rebuild only the necessary parts #33