Keymap conflicts with default `*-alt` keys

### Describe the bug
With the new keymap settings, a conflict will occur if the key I set is overlapped with the default `*-alt` keys

https://github.com/jesseduffield/lazygit/blob/68f3bcf53b0e19da3f7b1aaee19718605e339e8c/pkg/config/user_config.go#L955-L956

For example, if I set:

```yaml
prevItem: [i, ]
nextItem: [k, ]
```
With `MergeLegacyAltKeybindings` the `prevItem` will be merged and becomes `[i, , k]` and `nextItem` will be `[k, , j]`

As a result, the `k` will act as `prevItem`

I have to manually set:
```yaml
prevItem-alt:
nextItem-alt:
```
to disable the default `*-alt` keys

### To Reproduce
Steps to reproduce the behavior:

1. Set:
```yaml
prevItem: [i, ]
nextItem: [k, ]
```
2. Open `lazygit`
3. Goto the middle of some commits
4. Press `k` will move the cursor up

### Expected behavior
Press `k` will move the cursor down as configured

### Screenshots
NA
### Version info:
commit=, build date=, build source=Homebrew, version=0.62.2, os=darwin, arch=arm64, git version=2.50.1 (Apple Git-155)

### Terminal info:
kitty 0.46.2

### Additional context
NA

> [!NOTE]
> Please try updating to the latest version or [manually building](https://github.com/jesseduffield/lazygit/#manual) the latest `master` to see if the issue still occurs.