Reevalute size on window resize

I would like to have a terminal that always uses 40% of the available space.
I have the following config:

```lua
require("toggleterm").setup({
direction = "vertical",
size = function() return vim.o.columns * 0.4 end,
persist_size = false,
})
```

The config allows that, but only between terminal reopens like:

1. Terminal application window in fullscreen
2. Open toggleterm
3. its 40% width
4. Close toggleterm
5. Resize Terminal window
6. Reopen toggleterm
7. its 40% width of the the new Window size
8. This is as I like it

Not working is the following:

## current behavior

1. Terminal application window in fullscreen
2. Open toggleterm
3. its 40% width
4. Resize Terminal window
5. Toggleterm still has 40% width of the **old** window size

## desired behavior

I would like to see that on Window Resize also the size parameter gets reevaluated.

1. Terminal application window in fullscreen
2. Open toggleterm
3. its 40% width
4. Resize Terminal window
5. Toggleterm has 40% width of the **new** window size