Added "Substr" method to the "String" object.

Author: TiLied

CSharpToJavaScript/APIs/JS/Ecma/String.cs

@@ -16,7 +16,7 @@ public partial class String : StringPrototype
 	public static implicit operator string(String value) { return new String(value) { Value = value }; }
 
 	[To(ToAttribute.FirstCharToLowerCase)]
-	public StringPrototype Prototype { get; } = new();
+	public static StringPrototype Prototype { get; } = new();
 
 	[To(ToAttribute.FirstCharToLowerCase)]
 	public int Length { get; } = 0;
@@ -176,4 +176,10 @@ public string TrimStart()
 	{
 		throw new System.NotImplementedException();
 	}
+
+	//https://262.ecma-international.org/5.1/index.html#sec-B.2.3
+	public string Substr(int start, int length)
+	{
+		throw new System.NotImplementedException();
+	}
 }