Add shortcuts command

[Botan626]

Checklist

• [X] I read and understood ASF-ui's Contributing Guidelines
• [X] I also read FAQ
• [X] My idea doesn't duplicate existing ASF-ui functionality described on the wiki
• [X] I believe that my idea falls into ASF-ui's scope and should be offered as part of ASF-ui built-in functionality
• [X] This is not ASF suggestion

Enhancement purpose

With shortcuts command users who doesn't know about their existence could learn them without leaving it to a chance of accidentally finding out about them in the asf-ui wiki or users who forget them won't need to open asf-ui wiki and so on to read about them again.

So user will type sh on commands page, press Tab and Enter and will get quickly the list of available shortcuts.

Description for shortcuts command could be: List (or print) available shortcuts.

here this info could be expanded:

For a list of available commands type: commands
For a short description of a command type: help <command>
For a list of available shortcuts type: shortcuts