Add positioning choices to `GlIcon`

Right now, gl-icon box models are determined by their hierarchy instead of a property.

For example, if a gl-icon is inside of a gl-new-button, that icon has a right margin applied.

This should be controlled by the icon component itself.
For example <gl-icon icon="angle-right" line-position="left" /> would add the right margin, while line-position="right" would add left margin instead.

If we're concerned about internationalization, "left" and "right" could be "start" and "end" (which would have to be combined with other internationalization flagging to change which margin is applied).