stdlib: add support for bare const

Signed-off-by: Jos Verlinde <jos_verlinde@hotmail.com>

stubs/micropython-core/__builtins__.pyi

@@ -0,0 +1,27 @@
+# allows for type checking of additional builtins by pyright
+
+from typing import Tuple, TypeVar
+
+Const_T = TypeVar("Const_T", int, float, str, bytes, Tuple)  # constant
+
+def const(expr: Const_T) -> Const_T:
+    """
+    Used to declare that the expression is a constant so that the compiler can
+    optimise it.  The use of this function should be as follows::
+
+     from micropython import const
+
+     CONST_X = const(123)
+     CONST_Y = const(2 * CONST_X + 1)
+
+    Constants declared this way are still accessible as global variables from
+    outside the module they are declared in.  On the other hand, if a constant
+    begins with an underscore then it is hidden, it is not available as a global
+    variable, and does not take up any memory during execution.
+
+    This `const` function is recognised directly by the MicroPython parser and is
+    provided as part of the :mod:`micropython` module mainly so that scripts can be
+    written which run under both CPython and MicroPython, by following the above
+    pattern.
+    """
+    ...