Support latest package version by default when no version is specified in `#:package` directive

### Is your feature request related to a problem? Please describe.
Currently, when adding a NuGet package via the `#:package` directive in a file-based run, the version must be explicitly specified. For example:

```csharp
#:package Humanizer@2.*
```

or

```csharp
#:package Humanizer@*
```

These work fine, but if the version is omitted like this:

```csharp
#:package Humanizer
```

…it results in an error. This makes the initial user experience more complex, especially for beginners who are unfamiliar with versioning or just want to try out a package quickly.

### Describe the solution you'd like
It would be great if omitting the version in the `#:package` directive would default to the latest stable version available on NuGet. For example:

```csharp
#:package Humanizer
```

should internally resolve and use the latest version of the `Humanizer` package, just as if `@*` had been specified.

This change would:
- Make scripts easier to write and read.
- Lower the barrier to entry for newcomers.
- Align with common expectations from other package managers and tools.

### Additional context
This feature would provide a smoother onboarding experience and allow for quicker experimentation. Explicit versioning can still be used when needed, but shouldn't be required by default for simple scenarios.