setCustomElementsScopingSuffix = suffix => {
    if (!suffix.match(/^[a-zA-Z0-9_-]+$/)) {
      throw new Error("Only alphanumeric characters and dashes allowed for the scoping suffix");
    }
    suf = suffix;
  }
Sets the suffix to be used for custom elements scoping, f.e. pass "demo" to get tags such as "ui5-button-demo".
Note: by default all tags starting with "ui5-" will be scoped, unless you change this by calling "setCustomElementsScopingRules"

@public
@param suffix The scoping suffix