Fix casting of time points with non-system-clock duration with VS (#1748)

* Fix casting of time points with non-system-clock duration on Windows

Add explicit `time_point_cast` to time point with duration of system
clock. Fixes Visual Studio compile error.

* Add test case for custom time points casting
diff --git a/tests/test_chrono.cpp b/tests/test_chrono.cpp
index 195a93b..899d08d 100644
--- a/tests/test_chrono.cpp
+++ b/tests/test_chrono.cpp
@@ -14,6 +14,10 @@
 TEST_SUBMODULE(chrono, m) {
     using system_time = std::chrono::system_clock::time_point;
     using steady_time = std::chrono::steady_clock::time_point;
+
+    using timespan = std::chrono::duration<int64_t, std::nano>;
+    using timestamp = std::chrono::time_point<std::chrono::system_clock, timespan>;
+
     // test_chrono_system_clock
     // Return the current time off the wall clock
     m.def("test_chrono1", []() { return std::chrono::system_clock::now(); });
@@ -44,4 +48,8 @@
     // Float durations (issue #719)
     m.def("test_chrono_float_diff", [](std::chrono::duration<float> a, std::chrono::duration<float> b) {
         return a - b; });
+
+    m.def("test_nano_timepoint", [](timestamp start, timespan delta) -> timestamp {
+        return start + delta;
+    });
 }