The Persians were famed and feared for the horse archers they fielded. In 0 A.D., the Persians can train several cavalry types, but not horse archers: `pers_cavalry_archer_*` is actually a chariot.
The Persians did use scythed chariots, but only in small numbers; e.g. at the battle of Cunaxa (401 BC), Cyrus had 1600 horsemen and 20 chariots (i.e. 80:1) and Artaxerxes 6000 horsemen and 150 chariots (i.e. 40:1). Therefore let's keep it for a champion unit, like the other chariots in game (brit, maur, sele).
This patch:
* change the (citizen) `pers_cavalry_archer_*` <Actor> and <Icon> to the Seleucid horse archer's, instead of the Persian chariot's
* change it's generic and specific name to Parthian Horse Archer (Asabāra Parθava)
* create a new `pers_champion_chariot.xml` for the Babylonian Scythed Chariot (Raθa Bābiruviya), which uses the elite chariot <Actor> and <Icon>
The former is a cosmetic change, the latter is not trainable at any Persian structure (could be done in a future patch), therefore this patch does not alter gameplay balance.