Add ability to list plugins from CLI

Currently there is no CLI flag/command to view supported plugins, nor whether a plugin is enabled by default. This leads to confusion such as https://github.com/google/osv-scalibr/issues/618#issuecomment-4434542171, in which I saw `.csproj` file detection [was supported](https://github.com/google/osv-scalibr/blob/main/docs/supported_inventory_types.md#language-packages), but only if you explicitly enable it.

Perhaps a `plugin` command could solve this? For example:

```
$ osv-scanner plugin ls
javascript/packagejson (enabled)
dotnet/csproj (disabled)
```

Or, if `--experimental-plugins` is used without a value, it could list the available plugins:

```
$ osv-scanner --experimental-plugins
Available plugins:
...

$ osv-scanner --experimental-no-default-plugins --experimental-plugins
No plugins available
```