Surround selected text with a command input by the user

This is a follow up on #246 from @jlanza

The current surrounding command only enables the user to select a command from a fixed list. Allowing the user input would help. I am summarising the discussion in #246.

What could be done is to wrap the selections with {...} and put a multi-cursor before the opening {. Quoting @jlanza, other options could be

1. Something similar to the one you have already implemented. The only difference would be that in case the typed text is not in the list, it should be accepted. For instance, right now \ac for glossaries is not included in the list, so I cannot use the keyboard shortcut.
2. Another option would be to enable adding new commands to a configuration file, so you will be adding new commands to the list. I guess your current list is hard coded somewhere else. This way you won’t be typing something undesired. It is less friendly.
3. You can also think on a mix between option 1 and 2. That is, when you type and click enter on a non-included one, it is added to settings.json in your local folder.

[jlanza]

We already the user files for \newcommand, see the wiki.

Then, I guess just including this ones in the QuickPick window would be more than enough.

[tamuratak]

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