Core: Add DisplayServer flag for sharp corners

[Repiteo]

• Supersedes Disable rounded window corners on Windows #96575

Inspired by this comment, this PR aims to make the rounded/sharp corners found in Windows 11 a toggleable feature. One benefit of integrating with the existing flag system means that windows have the ability to individually choose whether their corners are rounded. Specifically targets the OS, so it doesn't do anything on embedded subwindows.

[Calinou]

Tested locally on Windows 11 23H2, it works as expected.

This makes more sense as a configurable flag, since non-game applications (and the Godot editor itself) will generally want to keep rounded corners to better fit with other apps on the system.

[Mickeon]

This makes more sense as a configurable flag, since non-game applications (and the Godot editor itself) will generally want to keep rounded corners to better fit with other apps on the system.

Makes you wonder if it should be set to true by default in the Project Settings and/or the Window node itself.

[Calinou]

Makes you wonder if it should be set to true by default in the Project Settings and/or the Window node itself.

I'm not sure whether we should intentionally diverge from Windows 11's intended look and feel by default here. It should be the topic of a separate proposal 🙂

[Repiteo]

I've considered it, but we don't have anything concrete on which users would prefer. We could have a poll sometime between dev3 & release on exactly that, changing the default value if sharp corners are overwhelmingly preferred, but that's obviously outside the scope of this PR.