Add 'system-ui' to font stack.

[bardiharborow]

Fixes #21354.

[klimashkin]

@bardiharborow Why system-ui is not first in list, why it should be after -apple-system?

[wolfy1339]

@klimashkin This was reverted in #22377 because​ it was causing issues

[GrimLink]

@mdo I don't understand why system-ui i has been removed from the stack.
Instead move the "Segoe UI" before the value in the stack instead.

Maybe like this:
$font-family-sans-serif: -apple-system, "Segoe UI", system-ui, ... !default;
This way we can still have the advantage of the system-ui without having the Windows bug.
Also BlinkMacSystemFont is an older version of system-ui for chrome on OSX.
Since Chrome 56 we should use the newer version, system-ui

[mdo]

@GrimLink Read #22328 for the background on why it was reverted outright. We didn't test, as far as I know, changing the font stack's order, but if you have a demo of it fixing the problems listed in the referenced issue, please open a new issue to discuss.