fix(lua): fix architecture-dependent behavior in usercmd "reg" (#20384)

I don't think using an integer as a NUL-terminated string can work on
big-endian systems, at least.

This is also not tested. Add a test.

Also fix a mistake in the docs of nvim_parse_cmd.

src/nvim/lua/executor.c

@@ -2036,7 +2036,8 @@ int nlua_do_ucmd(ucmd_T *cmd, exarg_T *eap, bool preview)
   }
   lua_setfield(lstate, -2, "fargs");
 
-  lua_pushstring(lstate, (const char *)&eap->regname);
+  char reg[2] = { (char)eap->regname, NUL };
+  lua_pushstring(lstate, reg);
   lua_setfield(lstate, -2, "reg");
 
   lua_pushinteger(lstate, eap->addr_count);