This PR fixes l10n issue when months names are being used in incorrect form in CupertinoDatePicker in CupertinoDatePickerMode.yearMonth (#130930).

The idea of this proposal is to add an optional parameter `standalone` for `CupertinoLocalizations.datePickerMonth` to be able to choose when to use months names in base form (intl DateSymbols.STANDALONEMONTHS) and when in day-dependent form (intl DateSymbols.MONTHS)

<details>
<summary>Before</summary>

<img width="366" alt="image" src="https://github.com/flutter/flutter/assets/32621121/1dd54fa7-6dd9-4053-889b-57134c145432">

<img width="387" alt="image" src="https://github.com/flutter/flutter/assets/32621121/c176070e-73e4-49d3-883b-ba31eca6d1d7">

</details>

<details>
<summary>After</summary>

<img width="369" alt="image" src="https://github.com/flutter/flutter/assets/32621121/255594f1-219d-4bd4-9b75-1012912f8ab0">

<img width="378" alt="image" src="https://github.com/flutter/flutter/assets/32621121/16bbb41f-3f62-4446-bf41-e27140b649a9">

</details>