Add .editorconfig to Core for specifying CA rules

[hartez]

Description of Change

For various reasons, the default code analysis rules have never been fully enforced in this repository. We would like to change that, but turning on all the rules for all projects at once is simply not an option with our current resources.

As an alternative, we can incrementally being enforcing these rules per-project, per-category, per-rule, and per-file via .editorconfig.

While it's tempting to mass-fix violations for a particular rule or category, doing so may generate changes across dozens or even hundreds of files at once. This makes for confusing change histories and conflicts. Instead, we can fix violations in smaller, easily reviewed subsets and prevent future violations in those areas using .editorconfig. By doing this incrementally, we can ratchet up the level of enforcement until we can turn it on for entire projects.

This change adds CA performance rule enforcement for Flex and PlatformTouchGraphicsView.