Suggestion: Use `defaultVhdSize` and add `defaultVhdType` to `.wslconfig`

I would like to propose changing the logic of WSL 2 commands and adding a VHD disk type setting.

To create a WSL 2 instance with a fixed disk size, the size must always be specified with `--vhd-size`, even if `.wslconfig` already contains a `defaultVhdSize` setting:

```ini
[wsl2]
defaultVhdSize=10GB
```

Example command:

```powershell
wsl --install Ubuntu-24.04 --vhd-size 10GB --fixed-vhd
```

The logical thing to do would be to use the `defaultVhdSize` setting from `.wslconfig` and simplify the command to:

```powershell
wsl --install Ubuntu-24.04 --fixed-vhd
```

It might also be a good idea to make a setting in `.wslconfig`, for example:

```ini
[wsl2]
defaultVhdType=fixed | dynamic
```

This would allow creating new instances with the desired disk type and size using just:

```powershell
wsl --install Ubuntu-24.04
```

The request concerns `.wslconfig` and the `wsl --install` path; start by tracing how `defaultVhdSize`, `--vhd-size`, and `--fixed-vhd` are currently handled. Define the precedence between command-line and configuration settings, then assess the proposed `defaultVhdType` behavior. Done means the documented install commands create new instances with the intended VHD size and type.

Written by the indexing model from the issue text.