fix(icon): set fill on host element and remove stroke

[brandyscarney]

This moves the fill property to the host element and removes the stroke since
A) I couldn't find any of our icons that even use stroke
B) I don't see any of the custom icons needing stroke a specific color by default
C) It allows the user to change these properties if desired on the <ion-icon> itself without needing CSS variables
D) It still inherits the color that gets set

This also updates the test to include custom icons.

This would fix ionic-team/ionic-framework#16483, but I need to do some testing in Ionic for it.

Below is a comparison between master and this branch, where the "Custom CSS" group is specifically setting the following:

ion-icon {
  stroke: red;
  fill: blue;
}

master	branch

[brandyscarney]

Got the ok on this from @bensperry 🆗