Add `themeOverride` view modifier

[rozd]

Override a whole theme

Assuming part of view tree requires look that different it from rest of the app:

PartnerView()
  .themeOverride(Theme.partner)

Implementation is simple and straightforward:

extension View {
      public func themeOverride(_ theme: Theme, animation: Animation? = nil) -> some View {
          environment(\.theme, theme)
      }
  }

Override a part of theme

When a view need several different styles, could be more valid use case:

PromoView()
  .themeOverride(\.colors.primaryColor, .init(light: .orange, dark: .yellow))

or

PromoView()
  .themeOverride(primaryColor: .init(light: .orange, dark: .yellow))

Implementation is complex, it would require either to convert let to var or generating an enormous number of methods – one for every style.