Add multiselect support for sequence command

[joshf26]

First off, thanks for making this extension. It's super helpful.

This change improves the multiselect support for the sequence command by making it more consistent with IntelliJ's String Manipulation plugin. I see the issue was already reported in #47.

This change adds a new multiselectData object that is passed to all number functions (although only sequence uses it currently) to allow data to be stored between calls. In this case, sequence uses it to keep track of the current number (offset) across selections. This allows the user to create a sequence of numbers in a column without affecting other numbers on each row. For example:

Becomes...

Let me know if you have any questions or feedback, thanks.