lua: add vim.register_keystroke_callback (#12536)

* feat: Add vim.register_keystroke_callback

* fixup: Forgot to remove mention of old option

* fixup: Answer jamessan comments

* fixup: Answer norcalli comments

* fixup: portability

* Update runtime/doc/lua.txt

Co-authored-by: Ashkan Kiani <ashkan.k.kiani@gmail.com>

src/nvim/lua/executor.c

@@ -1465,3 +1465,40 @@ void nlua_free_typval_dict(dict_T *const d)
     d->lua_table_ref = LUA_NOREF;
   }
 }
+
+void nlua_execute_log_keystroke(int c)
+{
+  char_u buf[NUMBUFLEN];
+  size_t buf_len = special_to_buf(c, mod_mask, false, buf);
+
+  lua_State *const lstate = nlua_enter();
+
+#ifndef NDEBUG
+  int top = lua_gettop(lstate);
+#endif
+
+  // [ vim ]
+  lua_getglobal(lstate, "vim");
+
+  // [ vim, vim._log_keystroke ]
+  lua_getfield(lstate, -1, "_log_keystroke");
+  luaL_checktype(lstate, -1, LUA_TFUNCTION);
+
+  // [ vim, vim._log_keystroke, buf ]
+  lua_pushlstring(lstate, (const char *)buf, buf_len);
+
+  if (lua_pcall(lstate, 1, 0, 0)) {
+    nlua_error(
+        lstate,
+        _("Error executing vim.log_keystroke lua callback: %.*s"));
+  }
+
+  // [ vim ]
+  lua_pop(lstate, 1);
+
+#ifndef NDEBUG
+  // [ ]
+  assert(top == lua_gettop(lstate));
+#endif
+}
+