Medusa System Added

Read about the medusa system [here](https://nadekobot.readthedocs.io/en/latest/medusa/creating-a-medusa/)

src/Nadeko.Medusa/Attributes/FilterAttribute.cs

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+namespace Nadeko.Snake;
+
+/// <summary>
+/// Overridden to implement custom checks which commands have to pass in order to be executed.
+/// </summary>
+[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class, AllowMultiple = true)]
+public abstract class FilterAttribute : Attribute
+{
+    public abstract ValueTask<bool> CheckAsync(AnyContext ctx);
+}

src/Nadeko.Medusa/Attributes/cmdAttribute.cs

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+namespace Nadeko.Snake;
+
+/// <summary>
+/// Marks a method as a snek command
+/// </summary>
+[AttributeUsage(AttributeTargets.Method)]
+public class cmdAttribute : Attribute
+{
+    /// <summary>
+    /// Command description. Avoid using, as cmds.yml is preferred
+    /// </summary>
+    public string? desc { get; set; }
+    
+    /// <summary>
+    /// Command args examples. Avoid using, as cmds.yml is preferred
+    /// </summary>
+    public string[]? args { get; set; }
+    
+    /// <summary>
+    /// Command aliases
+    /// </summary>
+    public string[] Aliases { get; }
+
+    public cmdAttribute()
+    {
+        desc = null;
+        args = null;
+        Aliases = Array.Empty<string>();
+    }
+    
+    public cmdAttribute(params string[] aliases)
+    {
+        Aliases = aliases;
+        desc = null;
+        args = null;
+    }
+}

src/Nadeko.Medusa/Attributes/injectAttribute.cs

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+namespace Nadeko.Snake;
+
+/// <summary>
+/// Marks services in command arguments for injection.
+/// The injected services must come after the context and before any input parameters.
+/// </summary>
+public class injectAttribute : Attribute
+{
+    
+}

src/Nadeko.Medusa/Attributes/leftoverAttribute.cs

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+namespace Nadeko.Snake;
+
+/// <summary>
+/// Marks the parameter to take 
+/// </summary>
+[AttributeUsage(AttributeTargets.Parameter)]
+public class leftoverAttribute : Attribute
+{
+    
+}

src/Nadeko.Medusa/Attributes/prioAttribute.cs

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+namespace Nadeko.Snake;
+
+/// <summary>
+/// Sets the priority of a command in case there are multiple commands with the same name but different parameters.
+/// Higher value means higher priority.
+/// </summary>
+[AttributeUsage(AttributeTargets.Method)]
+public class prioAttribute : Attribute
+{
+    public int Priority { get; }
+
+    /// <summary>
+    /// Snek command priority
+    /// </summary>
+    /// <param name="priority">Priority value. The higher the value, the higher the priority</param>
+    public prioAttribute(int priority)
+    {
+        Priority = priority;
+    }
+}

src/Nadeko.Medusa/Attributes/svcAttribute.cs

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+namespace Nadeko.Snake;
+
+/// <summary>
+/// Marks the class as a service which can be used within the same Medusa
+/// </summary>
+[AttributeUsage(AttributeTargets.Class)]
+public class svcAttribute : Attribute
+{
+    public Lifetime Lifetime { get; }
+    public svcAttribute(Lifetime lifetime)
+    {
+        Lifetime = lifetime;
+    }
+}
+
+/// <summary>
+/// Lifetime for <see cref="svcAttribute"/>
+/// </summary>
+public enum Lifetime
+{
+    Singleton,
+    Transient
+}