feat(lua): add vim._with

It's a function to perform operations in their own sealed context,
similar to pythons `with`. This helps ease operations where you need to
perform an operation in a specific context, and then restore the
context.

Marked as private for now as it's not ready for public use. The current
plan is to start using this internally so we can discover and fix any
problems. Once this is ready to be exposed it will be renamed to
`vim.with`.

Usage:

```lua
local ret = vim._with({context = val}, function()
  return "hello"
end)
```

, where `context` is any combination of:

- `buf`
- `emsg_silent`
- `hide`
- `horizontal`
- `keepalt`
- `keepjumps`
- `keepmarks`
- `keeppatterns`
- `lockmarks`
- `noautocmd`
- `options`
- `sandbox`
- `silent`
- `unsilent`
- `win`

(except for `win` and `buf` which can't be used at the same time). This
list will most likely be expanded in the future.

Work on https://github.com/neovim/neovim/issues/19832.

Co-authored-by: Lewis Russell <lewis6991@gmail.com>

src/nvim/ex_docmd.c

@@ -1729,12 +1729,6 @@ int execute_cmd(exarg_T *eap, CmdParseInfo *cmdinfo, bool preview)
   }
 
   const char *errormsg = NULL;
-#undef ERROR
-#define ERROR(msg) \
-  do { \
-    errormsg = msg; \
-    goto end; \
-  } while (0)
 
   cmdmod_T save_cmdmod = cmdmod;
   cmdmod = cmdinfo->cmdmod;
@@ -1745,16 +1739,19 @@ int execute_cmd(exarg_T *eap, CmdParseInfo *cmdinfo, bool preview)
   if (!MODIFIABLE(curbuf) && (eap->argt & EX_MODIFY)
       // allow :put in terminals
       && !(curbuf->terminal && eap->cmdidx == CMD_put)) {
-    ERROR(_(e_modifiable));
+    errormsg = _(e_modifiable);
+    goto end;
   }
   if (!IS_USER_CMDIDX(eap->cmdidx)) {
     if (cmdwin_type != 0 && !(eap->argt & EX_CMDWIN)) {
       // Command not allowed in the command line window
-      ERROR(_(e_cmdwin));
+      errormsg = _(e_cmdwin);
+      goto end;
     }
     if (text_locked() && !(eap->argt & EX_LOCK_OK)) {
       // Command not allowed when text is locked
-      ERROR(_(get_text_locked_msg()));
+      errormsg = _(get_text_locked_msg());
+      goto end;
     }
   }
   // Disallow editing another buffer when "curbuf->b_ro_locked" is set.
@@ -1802,7 +1799,6 @@ end:
 
   do_cmdline_end();
   return retv;
-#undef ERROR
 }
 
 static void profile_cmd(const exarg_T *eap, cstack_T *cstack, LineGetter fgetline, void *cookie)
@@ -2696,7 +2692,7 @@ int parse_command_modifiers(exarg_T *eap, const char **errormsg, cmdmod_T *cmod,
 
 /// Apply the command modifiers.  Saves current state in "cmdmod", call
 /// undo_cmdmod() later.
-static void apply_cmdmod(cmdmod_T *cmod)
+void apply_cmdmod(cmdmod_T *cmod)
 {
   if ((cmod->cmod_flags & CMOD_SANDBOX) && !cmod->cmod_did_sandbox) {
     sandbox++;