Fix ZWJ, ZWNJ issues

Current font load logic classifies ZWJ, ZWNJ as COMMON script.
This interrupts them from being combined into one emoji in the EMOJI + ZWJ + EMOJI case.

This patch treats ZWJ, ZWNJ as EMOJI script in this case,
so that they can be loaded with the same color emoji font.

Please refer the example below.
The expected result is to be combined into one emoji character due to ZWJ.
But the actual result is 3 characters rendered.

// example
TextLabel label = TextLabel::New("👩‍🔬");

Change-Id: Ic8e921ffcb63c27638afe781527b5c6b90aa92b1
Signed-off-by: Bowon Ryu <bowon.ryu@samsung.com>