Add tasks for all the new languages

Task/Abstract-type/Nit/abstract-type.nit

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+# Task: abstract type
+#
+# Methods without implementation are annotated `abstract`.
+#
+# Abstract classes and interfaces can contain abstract methods and concrete (i.e. non-abstract) methods.
+# Abstract classes can also have attributes.
+module abstract_type
+
+interface Inter
+	fun method1: Int is abstract
+	fun method2: Int do return 1
+end
+
+abstract class Abs
+	fun method1: Int is abstract
+	fun method2: Int do return 1
+	var attr: Int
+end