Implement dalamud-platform launch argument (#1452)

* implement dalamud platform launch arg
* implement cross-platform gamePath fallback
* refactor platform detection heuristic
* add cross platform dalamud runtime detection

Dalamud.Common/OSPlatformConverter.cs

@@ -0,0 +1,78 @@
+using System.Runtime.InteropServices;
+using Newtonsoft.Json;
+
+namespace Dalamud.Common;
+
+/// <summary>
+/// Converts a <see cref="OSPlatform"/> to and from a string (e.g. <c>"FreeBSD"</c>).
+/// </summary>
+public sealed class OSPlatformConverter : JsonConverter
+{
+    /// <summary>
+    /// Writes the JSON representation of the object.
+    /// </summary>
+    /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
+    /// <param name="value">The value.</param>
+    /// <param name="serializer">The calling serializer.</param>
+    public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
+    {
+        if (value == null)
+        {
+            writer.WriteNull();
+        }
+        else if (value is OSPlatform)
+        {
+            writer.WriteValue(value.ToString());
+        }
+        else
+        {
+            throw new JsonSerializationException("Expected OSPlatform object value");
+        }
+    }
+
+    /// <summary>
+    /// Reads the JSON representation of the object.
+    /// </summary>
+    /// <param name="reader">The <see cref="JsonReader"/> to read from.</param>
+    /// <param name="objectType">Type of the object.</param>
+    /// <param name="existingValue">The existing property value of the JSON that is being converted.</param>
+    /// <param name="serializer">The calling serializer.</param>
+    /// <returns>The object value.</returns>
+    public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
+    {
+        if (reader.TokenType == JsonToken.Null)
+        {
+            return null;
+        }
+        else
+        {
+            if (reader.TokenType == JsonToken.String)
+            {
+                try
+                {
+                    return OSPlatform.Create((string)reader.Value!);
+                }
+                catch (Exception ex)
+                {
+                    throw new JsonSerializationException($"Error parsing OSPlatform string: {reader.Value}", ex);
+                }
+            }
+            else
+            {
+                throw new JsonSerializationException($"Unexpected token or value when parsing OSPlatform. Token: {reader.TokenType}, Value: {reader.Value}");
+            }
+        }
+    }
+
+    /// <summary>
+    /// Determines whether this instance can convert the specified object type.
+    /// </summary>
+    /// <param name="objectType">Type of the object.</param>
+    /// <returns>
+    /// <c>true</c> if this instance can convert the specified object type; otherwise, <c>false</c>.
+    /// </returns>
+    public override bool CanConvert(Type objectType)
+    {
+        return objectType == typeof(OSPlatform);
+    }
+}