Fixes #290

The readme documents a way to customize the Material 3 top app bar provided by SkipUI's NavigationStack.

extension View {
    public func material3TopAppBar(_ options: @Composable (Material3TopAppBarOptions) -> Material3TopAppBarOptions) -> some View
}

public struct Material3TopAppBarOptions {
    public var title: @Composable () -> Void
    public var modifier: androidx.compose.ui.Modifier
    public var navigationIcon: @Composable () -> Void
    public var colors: androidx.compose.material3.TopAppBarColors
    public var scrollBehavior: androidx.compose.material3.TopAppBarScrollBehavior?
    public var preferCenterAlignedStyle: Bool
    public var preferLargeStyle: Bool
}

But one of these options is not like the others: scrollBehavior requires coordination between the top app bar and its Column container, via a modifier.nestedScroll(scrollBehavior.nestedScrollConnection).

Prior to this PR, the scroll connection was being passed to the TopAppBar after the default nestedScrollConnection was attached to the column's modifier, failing to connect the user-provided scrollBehavior's nestedScrollConnection to the Column. Now, we're extracting the user-provided scrollBehavior early, so we can pass its nestedScrollConnection to the column.

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