Add holding priority view modifier

[Wouter01]

Description

This PR adds a holdingPriority view modifier, to specify the NSSplitViewItem.holdingPriority property in SwiftUI:

func holdingPriority(_ priority: NSLayoutConstraint)) -> some View

It is applied in WorkspaceView, to prevent the terminal from resizing when resizing the window.

Checklist

• I read and understood the contributing guide as well as the code of conduct
• The issues this PR addresses are related to each other
• My changes generate no new warnings
• My code builds and runs on my machine
• My changes are all related to the related issue above
• I documented my code

Screenshots