Use text file icon for design rules

I'm not sure we need a custom icon for this as we don't
have an editor action that happens when you double-click
one of these files.  Someone's welcome to revisit this
later, though.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/7314

kicad/project_tree.cpp

@@ -77,6 +77,7 @@ PROJECT_TREE::PROJECT_TREE( PROJECT_TREE_PANE* parent ) :
     m_imageList->Add( KiBitmap( module_xpm ) );                 // TREE_FOOTPRINT_FILE
     m_imageList->Add( KiBitmap( library_xpm ) );                // TREE_SCHEMATIC_LIBFILE
     m_imageList->Add( KiBitmap( library_xpm ) );                // TREE_SEXPR_SYMBOL_LIB_FILE
+    m_imageList->Add( KiBitmap( editor_xpm ) );                 // DESIGN_RULES
 
     SetImageList( m_imageList );
 }

kicad/project_tree_pane.cpp

@@ -285,6 +285,7 @@ wxString PROJECT_TREE_PANE::GetFileExt( TREE_FILE_TYPE type )
     case TREE_FILE_TYPE::FOOTPRINT_FILE:        return KiCadFootprintFileExtension;
     case TREE_FILE_TYPE::SCHEMATIC_LIBFILE:     return LegacySymbolLibFileExtension;
     case TREE_FILE_TYPE::SEXPR_SYMBOL_LIB_FILE: return KiCadSymbolLibFileExtension;
+    case TREE_FILE_TYPE::DESIGN_RULES:          return DesignRulesFileExtension;
     default:                                    return wxEmptyString;
     }
 }

kicad/tree_file_type.h

@@ -58,6 +58,7 @@ enum class TREE_FILE_TYPE
     FOOTPRINT_FILE,        // footprint file (.kicad_mod)
     SCHEMATIC_LIBFILE,     // schematic library file (.lib)
     SEXPR_SYMBOL_LIB_FILE, // s-expression symbol library file (.kicad_sym)
+    DESIGN_RULES,          // design rules (.kicad_dru)
     MAX
 };