Add a static stubs directory

Stubs are mostly generated from modules in shared-bindings. But in order
to generate the stub the c code has to be commented with the appropriate
type hints. If a module is located somewhere else (like
micropython.const) and/or there are not the appropriate doc comments, no
stub is generated for it. For these occassions we can create static
stubs in the stubs directory that can be used in circuitpython-stubs.

This also creates a stub for micropython.const and adds the static
stubs directory into the makefile stubs command.

Author: dunkmann00

Makefile

@@ -280,6 +280,7 @@ stubs:
 	sed -e "s,__version__,`python -msetuptools_scm`," < setup.py-stubs > circuitpython-stubs/setup.py
 	cp README.rst-stubs circuitpython-stubs/README.rst
 	cp MANIFEST.in-stubs circuitpython-stubs/MANIFEST.in
+	cp -r stubs/* circuitpython-stubs
 	$(PYTHON) tools/board_stubs/build_board_specific_stubs/board_stub_builder.py
 	cp -r tools/board_stubs/circuitpython_setboard circuitpython-stubs/circuitpython_setboard
 	$(PYTHON) -m build circuitpython-stubs

stubs/micropython/__init__.pyi

@@ -0,0 +1,24 @@
+"""Access and control MicroPython internals"""
+
+from typing import TypeVar
+
+T = TypeVar("T")
+
+def const(expr: T) -> 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 micropython module mainly so that scripts can be
+    written which run under both CPython and MicroPython, by following the above
+    pattern."""