Add tile mixin

[bennypowers]

Styling tiles from consuming component works fine with native browsers, but polyfilled browsers do not respect those styles. This PR adds a mixin for slotted tile elements which works across browsers.

[vpusher]

@bennypowers The thing is you can use whatever tag as grid's tile. This is not limited to <tile>. So I an not really confortable at adding a mixin only for this tag.

By consuming component, you mean parent component that wraps the grid ?

[bennypowers]

yeah the consumer is the domHost of the-grid

In my case, I want to do something like

tile {
  display: flex;
  align-items: center;
  justify-content: center;
  font-family: Roboto, sans;
  font-size: 32px;
}

in the host. This breaks with shadycss