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);
+}