Fix #825 - Implement editor.quickOpen.caseSensitive setting

[bryphe]

Issue: The 'case-sensitivity' behavior of the QuickOpen is not very clear, and it is even less clear on how you can change the behavior to get preferred functionality (more discussion on this in #825).

Fix: This changes adds a new setting - editor.quickOpen.caseSensitive that accepts either a boolean value or smart. This change makes the pre-filter step case-agnostic, and then uses this setting for running the fusejs filter step.

[bryphe]

Fixes #825

[bryphe]

One issue I just realized is that the filtering logic is shared between Quick Open and the Command Palette. I think the behavior is reasonable, but it might influence how we name the configuration option (maybe it makes sense to be more general than just the quickOpen menu)

[bryphe]

I decided to change the setting to menu.caseSensitive, because this filtering logic is the same across all menus - Quick Open, Command Palette, and (soon) symbol search through language servers.