Add head, tail, initial snippets

snippets/head.md

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+---
+title: head
+tags: list,beginner
+---
+
+Returns the head of a list.
+
+use `lst[0]` to return the first element of the passed list.
+
+```py
+def head(lst):
+  return lst[0]
+```
+
+```py
+head([1, 2, 3]); # 1
+```

snippets/initial.md

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+---
+title: initial
+tags: list,beginner
+---
+
+Returns all the elements of a list except the last one.
+
+Use `lst[0:-1]` to return all but the last element of the list.
+
+```py
+def initial(lst):
+  return lst[0:-1]
+```
+
+```py
+initial([1, 2, 3]); # [1,2]
+```

snippets/tail.md

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+---
+title: tail
+tags: list,beginner
+---
+
+Returns all elements in a list except for the first one.
+
+Return `lst[1:]` if the list's length is more than `1`, otherwise, return the whole list.
+
+```py
+def tail(lst):
+  return lst[1:] if len(lst) > 1 else lst
+```
+
+```py
+tail([1, 2, 3]); # [2,3]
+tail([1]); # [1]
+```