Add 'Save As' functionality

[adeeteya]

This pull request adds a "Save As" feature to the application, allowing users to save the current file under a new name or location. It introduces a new menu item, updates the save logic to distinguish between saving an opened file and saving as a new file, and ensures the UI and localization support the new functionality.

Feature Addition: "Save As" Functionality

• Added a new MenuItem.saveAs to the MenuItem enum and integrated it into the menu UI, allowing users to select "Save As" from the menu. [1] [2]
• Implemented the _saveFileAs method, which prompts the user to choose a file name and location, and updates state accordingly.
• Updated _saveFile to save directly if a file is already opened, or invoke "Save As" if not, providing user feedback on success or failure.
• Modified the menu action handler to support the new "Save As" option.

State Management Improvements

• Introduced _isOpenedFile to track whether the current file was opened from disk, and updated this state appropriately when opening, saving, or clearing files. [1] [2] [3] [4]

Localization

• Added the "Save As" string and its description to the localization file for English (app_en.arb).