command `Mark.Get-Command` / `Mark.Get-Help`

- display table to summarize commands
- can filter by property

```ps1
Mark.Get-Command # or Mark.Get-Help
```
| Command | Language | Description |
|--------|--------|--------|
| `Mark.Md.Header` | `Markdown`| synopsis text ... |
| `Mark.Html.Header` | `Html`| synopsis text ... | 
