✨ Setting to change keyboard shortcut to add multiple cursors

[austincondiff]

Description

It is now possible to add multiple cursors with ⌃⇧ Click, thanks to STTextView for providing it, which aligns with Xcode. This is what other editors use:

Editor	Shortcut
Xcode	⌃⇧ Click
VS Code	⌥ Click
Atom	⌘ Click
IntelliJ	⌥ ⇧ Click

Users will probably want a setting to change this shortcut so we should provide a way for them to do this.