objstr: Add .splitlines() method.

splitlines() occurs ~179 times in CPython3 standard library, so was
deemed worthy to implement. The method has subtle semantic differences
from just .split("\n"). It is also defined as working for any end-of-line
combination, but this is currently not implemented - it works only with
LF line-endings (which should be OK for text strings on any platforms,
but not OK for bytes).

py/qstrdefs.h

@@ -299,6 +299,11 @@ Q(find)
 Q(rfind)
 Q(rindex)
 Q(split)
+#if MICROPY_PY_BUILTINS_STR_SPLITLINES
+Q(splitlines)
+Q(keepends)
+Q(\n)
+#endif
 Q(rsplit)
 Q(startswith)
 Q(endswith)