fix(grid): working check for CSS custom variables in Safari

[hal1984]

Short description of what this resolves:

Safari ignores alternate values for --ion-grid-columns when determining col width, reverting always to the default 12.

Changes proposed in this pull request:

• Use a equivalent way of calling CSS.supports() that works with custom variables in Safari, cause at the moment is partially broken in WebKit: https://bugs.webkit.org/show_bug.cgi?id=154669

Ionic Version: 4.0-beta.3

More details here:

#14562 (comment)