Allow custom sizes in SidebarItemsConfiguration

[plarson]

First PR for this repository. I needed to make the sidebar icons bigger so I reconfigured SidebarItemsConfiguration to not be private.

Pre-launch Checklist

• I have incremented the package version as appropriate and updated CHANGELOG.md with my changes
• I have added/updated relevant documentation
• I have run "optimize/organize imports" on all changed files
• I have addressed all analyzer warnings as best I could

[GroovinChip]

Hi @plarson and thank you for your first PR to this project.

I am not certain that making SidebarItemsConfiguration is the best technical choice to achieve the desired result. I'd lean towards adding an iconSize property to the Sidebar itself.

That said, it is not clear to me if Apple has a design opinion about this. The Human Interface Guidelines make no mention of it, and research is required to see if SwiftUI supports it. I will look into this and get back to you.

[GroovinChip]

@plarson I have the answer: https://developer.apple.com/design/human-interface-guidelines/sidebars#macOS

Please look through this and update your PR to match the specs outlined here. Thanks!