New encoding in 4123: "Chinese (GB18030)"

[FichteFoll]

To be included in our encodings map.

Exact name via https://discord.com/channels/280102180189634562/280157083356233728/917661821031309313.

[jfcherng]

As per bschaaf's saying in https://discord.com/channels/280102180189634562/280157067396775936/1364824222966485022

Note this is missing GB18030, CP 862 and Windows 865