Custom step insertion is error-prone

It's quite easy to make mistakes with the custom step MSBuild/command-line and insert them in an unintended order. For example you might write
```xml
<_TrimmerCustomSteps Include="$(_TaskAsm)" AfterStep="OutputStep" Type="Xamarin.Linker.RegistrarStep" />
<_TrimmerCustomSteps Include="$(_TaskAsm)" AfterStep="OutputStep" Type="Xamarin.Linker.DoneStep" />
```
expecting that `DoneStep` will come last when really this just determines the order of insertions (this actually was the case before https://github.com/mono/linker/pull/2082). Maybe it would be better to provide a way to insert multiple custom steps at once in a fixed order, similar to SubStepDispatcher (see https://github.com/xamarin/xamarin-android/blob/main/src/Microsoft.Android.Sdk.ILLink/PreserveSubStepDispatcher.cs#L9-L17 for example), but with a list of Steps instead of SubSteps.

This probably isn't worth it just for xamarin-android/xamarin-macios, but we should consider improving this if we ever want the plugin model to become more widely used.