This PR adds menu item for `workspace::OpenFiles` in app menu on Linux
and Windows.

Context:
When opening a file or folder on Linux and Windows via the native file
picker, the picker can be either in file-only mode or folder-only mode.
This means you have to open it already knowing whether you want to open
a file or a folder, unlike macOS, which lets you choose either in the
same picker.

For this reason, a new action, `workspace::OpenFiles`, was recently
added for Linux and Windows. This is basically file-only mode, alongside
the existing `workspace::Open` action, which is folder-only. In macOS,
the `workspace::Open` action is sufficient to open both file and folder.

Before:  
<img
src="https://github.com/user-attachments/assets/67dc95d6-e98d-438a-9568-570e87617f85"
alt="Before" width="200" />

After:  
<img
src="https://github.com/user-attachments/assets/d0ffd02c-0f48-4edc-b426-4d430f2e0c86"
alt="After" width="200" />

Release Notes:

- Added "Open File" action in file menu for Linux and Windows.