Unify time modification on SD and SPIFFS (#738)

* Add access to last write date time

add example

* rename cpp to ino orz

* wrong copy -past

* No comment orz

* Add missing space

libraries/FS/src/vfs_api.h

@@ -67,6 +67,7 @@ public:
     size_t      size() const override;
     void        close() override;
     const char* name() const override;
+    time_t getLastWrite()  override;
     boolean     isDirectory(void) override;
     FileImplPtr openNextFile(const char* mode) override;
     void        rewindDirectory(void) override;