autoinsert: add multiple cursors support

[loremdifr]

adds the functionality for multiple cursors as well as matching the behavior of sublime/vscode more closely.

changes in behavior:

• after wrapping, always ends with the cursor at the end of the selection, to match sublime/vscode behavior
• making the ctrl+shift+backspace command delete to the start of the line instead of just duplicating the binding of ctrl+backspace

[jgmdev]

This is working great, thanks a lot!