Add config option to customize placeholder brackets

Example

ClassicEditor.create(document.querySelector("#editor"), {
  plugins: [
    // ...
    Placeholder,
  ],
  toolbar: [
    // ...
    "placeholder",
  ],
  placeholderProps: {
    types: ["First Name", "Date"],
  },
  placeholderBrackets: {
    open: "{",
    close: "}",
  }
});

Thanks to @baspeeters for contribution!