Add private field modifier to classes

[vieira]

Add syntax highlighting to the private field modifier # as defined in tc39/proposal-class-fields, tc39/proposal-static-class-features and tc39/proposal-private-methods.

export class JSDOM {
  static #red = "#ff0000";
  #createdBy;
  
  #registerWithRegistry() {
    // ... elided ...
  }

  static #finalizeFactoryCreated(jsdom, factoryName) {
    jsdom.#createdBy = factoryName;
    jsdom.#registerWithRegistry(registry);
    return jsdom;
  }
}