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/space_api/CMakeLists.txt

@@ -50,6 +50,7 @@ set(LIB
   bf_editor_space_outliner
   bf_editor_space_script
   bf_editor_space_sequencer
+  bf_editor_space_spreadsheet
   bf_editor_space_statusbar
   bf_editor_space_text
   bf_editor_space_topbar

source/blender/editors/space_api/spacetypes.c

@@ -95,6 +95,7 @@ void ED_spacetypes_init(void)
   ED_spacetype_clip();
   ED_spacetype_statusbar();
   ED_spacetype_topbar();
+  ED_spacetype_spreadsheet();
 
   /* Register operator types for screen and all spaces. */
   ED_operatortypes_userpref();