Define mypy_extensions.i16 in stubs (#15562)

I forgot to add this when I added mypyc support for the i16 native
integer type.

The mypy_extensions stubs are maintained here instead of typeshed.

This is a copy of the stubs for `i32`, with references to `i32` replaced
with `i16`.

mypy/typeshed/stubs/mypy-extensions/mypy_extensions.pyi

@@ -146,3 +146,38 @@ class i32:
     def __ge__(self, x: i32) -> bool: ...
     def __gt__(self, x: i32) -> bool: ...
     def __index__(self) -> int: ...
+
+class i16:
+    @overload
+    def __new__(cls, __x: str | ReadableBuffer | SupportsInt | SupportsIndex | SupportsTrunc = ...) -> i16: ...
+    @overload
+    def __new__(cls, __x: str | bytes | bytearray, base: SupportsIndex) -> i16: ...
+
+    def __add__(self, x: i16) -> i16: ...
+    def __radd__(self, x: i16) -> i16: ...
+    def __sub__(self, x: i16) -> i16: ...
+    def __rsub__(self, x: i16) -> i16: ...
+    def __mul__(self, x: i16) -> i16: ...
+    def __rmul__(self, x: i16) -> i16: ...
+    def __floordiv__(self, x: i16) -> i16: ...
+    def __rfloordiv__(self, x: i16) -> i16: ...
+    def __mod__(self, x: i16) -> i16: ...
+    def __rmod__(self, x: i16) -> i16: ...
+    def __and__(self, x: i16) -> i16: ...
+    def __rand__(self, x: i16) -> i16: ...
+    def __or__(self, x: i16) -> i16: ...
+    def __ror__(self, x: i16) -> i16: ...
+    def __xor__(self, x: i16) -> i16: ...
+    def __rxor__(self, x: i16) -> i16: ...
+    def __lshift__(self, x: i16) -> i16: ...
+    def __rlshift__(self, x: i16) -> i16: ...
+    def __rshift__(self, x: i16) -> i16: ...
+    def __rrshift__(self, x: i16) -> i16: ...
+    def __neg__(self) -> i16: ...
+    def __invert__(self) -> i16: ...
+    def __pos__(self) -> i16: ...
+    def __lt__(self, x: i16) -> bool: ...
+    def __le__(self, x: i16) -> bool: ...
+    def __ge__(self, x: i16) -> bool: ...
+    def __gt__(self, x: i16) -> bool: ...
+    def __index__(self) -> int: ...