windows: Improve default search path.

The default value for MICROPYPATH used in unix/main.c is
"~/.micropython/lib:/usr/lib/micropython" which has 2 problems when used in
the Windows port:
- it has a ':' as path separator but the port uses ';' so the entire string
  is effectively discarded since it gets interpreted as a single path which
  doesn't exist
- /usr/lib/micropython is not a valid path in a standard Windows
  environment

Override the value with a suitable default.

docs/unix/quickref.rst

@@ -82,8 +82,9 @@ Environment variables
 .. envvar:: MICROPYPATH
 
     Overrides the default search path for MicroPython libraries. ``MICROPYPATH``
-    should be set to a colon separated list of directories. If ``MICROPYPATH`` is
-    not defined, the search path will be ``~/.micropython/lib:/usr/lib/micropython``
+    should be set to a colon (semicolon for Windows port) separated list of
+    directories. If ``MICROPYPATH`` is not defined, the search path will be
+    ``~/.micropython/lib:/usr/lib/micropython`` (``~/.micropython/lib`` for Windows port)
     or the value of the ``MICROPY_PY_SYS_PATH_DEFAULT`` option if it was set
     when MicroPython itself was compiled.
 

ports/windows/mpconfigport.h

@@ -81,6 +81,9 @@
 #define MICROPY_PY_BUILTINS_SLICE_ATTRS (1)
 #define MICROPY_PY_SYS_EXIT         (1)
 #define MICROPY_PY_SYS_PLATFORM     "win32"
+#ifndef MICROPY_PY_SYS_PATH_DEFAULT
+#define MICROPY_PY_SYS_PATH_DEFAULT "~/.micropython/lib"
+#endif
 #define MICROPY_PY_SYS_MAXSIZE      (1)
 #define MICROPY_PY_SYS_STDFILES     (1)
 #define MICROPY_PY_SYS_EXC_INFO     (1)