Icons: Add severity icons to the design kit

[andrew-ronaldson]

Goal

Follow up to patternfly/patternfly-design#1219

### Tasks
- [x] Create the final SVG icons
- [x] Add icons to the iconWrapper in v6 kit

@smckinno proposed severity icons which are separated from the standard alerts we offer in PatternFly. This new set of icons covers several levels of severity.