perf: replace StringBuilder with ValueStringBuilder

Author: TimothyMakkison

src/Refit/SeparatedCaseFormatter.cs

@@ -23,22 +23,22 @@ internal static string Format(string key, char separator)
             return key;
         }
 
-        var builder = new StringBuilder(key.Length + SeparatorCapacityHeadroom);
+        var builder = new ValueStringBuilder(key.Length + SeparatorCapacityHeadroom);
         for (var i = 0; i < key.Length; i++)
         {
             var current = key[i];
             if (char.IsUpper(current))
             {
                 if (i > 0 && NeedsSeparatorBefore(key, i))
                 {
-                    _ = builder.Append(separator);
+                    builder.Append(separator);
                 }
 
-                _ = builder.Append(char.ToLowerInvariant(current));
+                builder.Append(char.ToLowerInvariant(current));
             }
             else
             {
-                _ = builder.Append(current);
+                builder.Append(current);
             }
         }