refactor(options)!: unify set_option and set_string_option

While the interfaces for setting number and boolean options are now unified by #25394, there is still a separate `set_string_option` function that is used for setting a string option. This PR removes that function and merges it with set_option.

BREAKING CHANGE: `v:option_old` is now the old global value for all global-local options, instead of just string global-local options. Local value for a global-local number/boolean option is now unset when the option is set (e.g. using `:set` or `nvim_set_option_value`) without a scope, which means they now behave the same way as string options.

Ref: #25672

runtime/doc/autocmd.txt

@@ -778,11 +778,10 @@ OptionSet			After setting an option (except during
 				This does not set |<abuf>|, you could use
 				|bufnr()|.
 
-				Note that when setting a |global-local| string
-				option with |:set|, then |v:option_old| is the
-				old global value. However, for all other kinds
-				of options (local string options, global-local
-				number options, ...) it is the old local
+				Note that when setting a |global-local| option
+				with |:set|, then |v:option_old| is the old
+				global value. However, for all options that
+				are not global-local it is the old local
 				value.
 
 				OptionSet is not triggered on startup and for

runtime/doc/news.txt

@@ -73,7 +73,12 @@ The following changes may require adaptations in user config or plugins.
 
 • |OptionSet| autocommand args |v:option_new|, |v:option_old|,
   |v:option_oldlocal|, |v:option_oldglobal| now have the type of the option
-  instead of always being strings.
+  instead of always being strings. |v:option_old| is now the old global value
+  for all global-local options, instead of just string global-local options.
+
+• Local value for a global-local number/boolean option is now unset when
+  the option is set (e.g. using |:set| or |nvim_set_option_value()|) without a
+  scope, which means they now behave the same way as string options.
 
 ==============================================================================
 NEW FEATURES                                                    *news-features*

runtime/doc/vim_diff.txt

@@ -289,6 +289,10 @@ Normal commands:
   |Q| replays the last recorded macro instead of switching to Ex mode (|gQ|).
 
 Options:
+  Local values for global-local number/boolean options are unset when the
+  option is set without a scope (e.g. by using |:set|), similarly to how
+  global-local string options work.
+
   'autoread'    works in the terminal (if it supports "focus" events)
   'cpoptions'   flags: |cpo-_|
   'diffopt'     "linematch" feature
@@ -381,7 +385,8 @@ Variables:
   |v:windowid| is always available (for use by external UIs)
   |OptionSet| autocommand args |v:option_new|, |v:option_old|,
   |v:option_oldlocal|, |v:option_oldglobal| have the type of the option
-  instead of always being strings.
+  instead of always being strings. |v:option_old| is now the old global value
+  for all global-local options, instead of just string global-local options.
 
 Vimscript:
   |:redir| nested in |execute()| works.

src/nvim/api/deprecated.c

@@ -681,21 +681,6 @@ static void set_option_to(uint64_t channel_id, void *to, OptReqScope req_scope,
     return;
   });
 
-  if (value.type == kObjectTypeNil) {
-    if (req_scope == kOptReqGlobal) {
-      api_set_error(err, kErrorTypeException, "Cannot unset option '%s'", name.data);
-      return;
-    } else if (!(flags & SOPT_GLOBAL)) {
-      api_set_error(err, kErrorTypeException,
-                    "Cannot unset option '%s' because it doesn't have a global value",
-                    name.data);
-      return;
-    } else {
-      unset_global_local_option(name.data, to);
-      return;
-    }
-  }
-
   bool error = false;
   OptVal optval = object_as_optval(value, &error);
 

src/nvim/eval.c

@@ -7229,12 +7229,11 @@ void set_vim_var_dict(const VimVarIndex idx, dict_T *const val)
 /// Set v:variable to tv.
 ///
 /// @param[in]  idx  Index of variable to set.
-/// @param[in,out]  val  Value to set to. Reference count will be incremented.
-///                      Also keys of the dictionary will be made read-only.
+/// @param[in]  val  Value to set to. Will be copied.
 void set_vim_var_tv(const VimVarIndex idx, typval_T *const tv)
 {
   tv_clear(&vimvars[idx].vv_di.di_tv);
-  vimvars[idx].vv_di.di_tv = *tv;
+  tv_copy(tv, &vimvars[idx].vv_di.di_tv);
 }
 
 /// Set the v:argv list.