Introduce pybind11::detail::is_move_constructible (#4631)

To support the use case captured in the new test_vector_unique_ptr_member.cpp

tests/test_vector_unique_ptr_member.py

@@ -0,0 +1,14 @@
+import pytest
+
+from pybind11_tests import vector_unique_ptr_member as m
+
+
+@pytest.mark.parametrize("num_elems", range(3))
+def test_create(num_elems):
+    vo = m.VectorOwner.Create(num_elems)
+    assert vo.data_size() == num_elems
+
+
+def test_cast():
+    vo = m.VectorOwner.Create(0)
+    assert m.py_cast_VectorOwner_ptr(vo) is vo