Medusa System Added

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

src/Nadeko.Medusa/ParamParser/ParamParser.cs

@@ -0,0 +1,16 @@
+namespace Nadeko.Snake;
+
+/// <summary>
+/// Overridden to implement parsers for custom types 
+/// </summary>
+/// <typeparam name="T">Type into which to parse the input</typeparam>
+public abstract class ParamParser<T>
+{
+    /// <summary>
+    /// Overridden to implement parsing logic
+    /// </summary>
+    /// <param name="ctx">Context</param>
+    /// <param name="input">Input to parse</param>
+    /// <returns>A <see cref="ParseResult{T}"/> with successful or failed status</returns>
+    public abstract ValueTask<ParseResult<T>> TryParseAsync(AnyContext ctx, string input);
+}

src/Nadeko.Medusa/ParamParser/ParseResult.cs

@@ -0,0 +1,48 @@
+namespace Nadeko.Snake;
+
+public readonly struct ParseResult<T>
+{
+    /// <summary>
+    /// Whether the parsing was successful
+    /// </summary>
+    public bool IsSuccess { get; private init; }
+    
+    /// <summary>
+    /// Parsed value. It should only have value if <see cref="IsSuccess"/> is set to true
+    /// </summary>
+    public T? Data { get; private init;  }
+
+    /// <summary>
+    /// Instantiate a **successful** parse result
+    /// </summary>
+    /// <param name="data">Parsed value</param>
+    public ParseResult(T data)
+    {
+        Data = data;
+        IsSuccess = true;
+    }
+    
+
+    /// <summary>
+    /// Create a new <see cref="ParseResult{T}"/> with IsSuccess = false
+    /// </summary>
+    /// <returns>A new <see cref="ParseResult{T}"/></returns>
+    public static ParseResult<T> Fail()
+        => new ParseResult<T>
+        {
+            IsSuccess = false,
+            Data = default,
+        };
+
+    /// <summary>
+    /// Create a new <see cref="ParseResult{T}"/> with IsSuccess = true
+    /// </summary>
+    /// <param name="obj">Value of the parsed object</param>
+    /// <returns>A new <see cref="ParseResult{T}"/></returns>
+    public static ParseResult<T> Success(T obj)
+        => new ParseResult<T>
+        {
+            IsSuccess = true,
+            Data = obj,
+        };
+}