Spreadsheet: add boilerplate code for new editor type

This adds the initial boilerplate code that is required to introduce
the new spreadsheet editor. The editor is still hidden from the ui.

It can be made visible by undoing the change in `rna_screen.c`.

This patch does not contain any business logic for the spreadsheet editor.

Differential Revision: https://developer.blender.org/D10645

Ref T86279.

source/blender/editors/include/ED_space_api.h

@@ -55,6 +55,7 @@ void ED_spacetype_userpref(void);
 void ED_spacetype_clip(void);
 void ED_spacetype_statusbar(void);
 void ED_spacetype_topbar(void);
+void ED_spacetype_spreadsheet(void);
 
 /* calls for instancing and freeing spacetype static data
  * called in WM_init_exit */