Print the init-only marker after the set keyword again

[siegfriedpammer]

When init accessors are unavailable (e.g. -lv CSharp8_0, or the setting turned off), an
init-only setter decompiles as set plus an /*init*/ marker. The marker used to sit right
after the keyword; since the slot AST migration it lands after the accessor body:

// 10.1                                     // master
public int Auto { get; set/*init*/; }       public int Auto { get; set; /*init*/}
set/*init*/                                 set
{                                           {
    backing = value;                            backing = value;
}                                           }
                                            /*init*/}

Comments used to be child nodes that InsertSpecialsDecorator flushed when the next node
started printing; as trailing trivia they are printed in EndNode. Moving the marker to the
body's leading trivia would not work either - the auto-property transform drops the body and
would take the marker with it - so the accessor itself carries the init-only fact now and the
printer writes the marker next to the keyword. The flag is [ExcludeFromMatch], because a
pattern written for a setter has to keep matching init-only setters.

Verified against ilspycmd built from release/10.1: identical output again. Covered by a new
Ugly fixture that decompiles with C# 8 settings (red before, green after); the rest of
ICSharpCode.Decompiler.Tests stays green (3333 passed, roundtrip excluded).

🤖 Generated with Claude Code