[JP, TC, and future SC] Add 2em dash and its ligature

[VentusUta]

Japanese and Chinese use 2em dash as em dash in English, which need always be connected.

Maybe we should add following characters in JP, TC, and future SC version.

• U+2E3A ⸺
• Ligature of two U+2014 ——
• Ligature of two U+2015 ――

For reference, this is how they render in Noto Sans CJK:

[NightFurySL2001]

This problem still persists in the SC version.