Static icon classes and its members should be const
Hello, thanks for the great set of icons!
 
I've noticed that different types of icons have been put into separate classes as static members of the abstract class `PhosphorIcons` since I've upgraded to version `2.0.0` which is really nice.

My latest builds failed for a reason that I believe is related to the way PhosphorIcons are accessed.
`Error (Xcode): This application cannot tree shake icons fonts. It has non-constant instances of IconData at the following locations:
`

Since each class and its members are auto generated and do not change at any time I believe both classes such as `PhosphorIconsBold()` and all of it's members could and should be const. This way each icon could be accessed in a constant way which improves performance.