refactor(options): remove getoption_T and introduce OptVal (#23850)

Removes the `getoption_T` struct and also introduces the `OptVal` struct
to unify the methods of getting/setting different option value types.
This is the first of many PRs to reduce code duplication in the Vim
option code as well as to make options easier to maintain. It also
increases the flexibility and extensibility of options. Which opens the
door for things like Array and Dictionary options.

src/nvim/api/deprecated.c

@@ -709,14 +709,13 @@ static void set_option_to(uint64_t channel_id, void *to, int type, String name,
     }
   }
 
-  long numval = 0;
-  char *stringval = NULL;
+  OptVal optval = NIL_OPTVAL;
 
   if (flags & SOPT_BOOL) {
     VALIDATE(value.type == kObjectTypeBoolean, "Option '%s' value must be Boolean", name.data, {
       return;
     });
-    numval = value.data.boolean;
+    optval = BOOLEAN_OPTVAL(value.data.boolean);
   } else if (flags & SOPT_NUM) {
     VALIDATE(value.type == kObjectTypeInteger, "Option '%s' value must be Integer", name.data, {
       return;
@@ -725,12 +724,12 @@ static void set_option_to(uint64_t channel_id, void *to, int type, String name,
              "Option '%s' value is out of range", name.data, {
       return;
     });
-    numval = (int)value.data.integer;
+    optval = NUMBER_OPTVAL(value.data.integer);
   } else {
     VALIDATE(value.type == kObjectTypeString, "Option '%s' value must be String", name.data, {
       return;
     });
-    stringval = value.data.string.data;
+    optval = STRING_OPTVAL(value.data.string);
   }
 
   // For global-win-local options -> setlocal
@@ -739,6 +738,6 @@ static void set_option_to(uint64_t channel_id, void *to, int type, String name,
                         (type == SREQ_GLOBAL) ? OPT_GLOBAL : OPT_LOCAL;
 
   WITH_SCRIPT_CONTEXT(channel_id, {
-    access_option_value_for(name.data, &numval, &stringval, opt_flags, type, to, false, err);
+    set_option_value_for(name.data, optval, opt_flags, type, to, err);
   });
 }