Help: Add notes for topic 'get_filename_component_last_ext'

Help/release/dev/get_filename_component_last_ext.rst

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+get_filename_component_last_ext
+-------------------------------
+
+* The :command:`get_filename_component` command gained new
+  ``LAST_EXT`` and ``NAME_WLE`` variants to work with the
+  extension after the last ``.`` in the name.