Add ToHex method to Argb32

[lindexi]

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Description

Add ToHex method to Argb32.

The Argb32 will use the AARRGGBB format. Which format is what the WPF and WinUI and MAUI and Avalonia and so on used.

See #2962

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[gfoidl]

This allocates more than needed. Instead the string.Create-overload could be used, where you can pass in a state and operate on the string-buffer directly, thus avoiding any intermediate allocation.

Although for .NET 10 the JIT will maybe stack-alloc the intermediate array allocation, but I'm not sure if this is actually the case here.