[V1] Icon refactor

[veekeys]

• [x] Move to helpers instead of components and restructure the variables.

• [ ] Mayank wants to do something with web components and custom selector in there, so we could target those directly instead of first level element.

• [x] Remove color from icon sizes mixins.

• [ ] Use current color maybe.

[mayank99]

To expand, I was thinking three things:

1. All our svgs in @itwin/itwinui-icons should come with width, height, and fill attributes. This would mean we have to write less CSS.
2. All the icons in @itwin/itwinui-icons-elements should have a way to target them with a common selector. This could be a data attribute, e.g. data-iui-icon=true, or even add the name of the icon: data-iui-icon=calendar
3. Assuming those changes are made in the icons packages, in @itwin/itwinui-css we could target all web components with [data-iui-icon] and apply any special styles, e.g. display: flex.