patch 8.2.3562: cannot add color names

Problem:    Cannot add color names.
Solution:   Add the v:colornames dictionary. (Drew Vogel, closes #8761)

src/evalvars.c

@@ -149,6 +149,7 @@ static struct vimvar
     {VV_NAME("argv",		 VAR_LIST), VV_RO},
     {VV_NAME("collate",		 VAR_STRING), VV_RO},
     {VV_NAME("exiting",		 VAR_SPECIAL), VV_RO},
+    {VV_NAME("colornames",       VAR_DICT), VV_RO},
 };
 
 // shorthand
@@ -248,6 +249,8 @@ evalvars_init(void)
 
     set_vim_var_nr(VV_ECHOSPACE,    sc_col - 1);
 
+    set_vim_var_dict(VV_COLORNAMES, dict_alloc());
+
     // Default for v:register is not 0 but '"'.  This is adjusted once the
     // clipboard has been setup by calling reset_reg_var().
     set_reg_var(0);