By Zahra Matarani and Joseph Campbell TAIPEI/TAINAN, Taiwan (Reuters) – Taiwan’s three main political parties will hold mass rallies on Friday evening in a final push for support ahead of critical presidential and parliamentary elections happening against a backdrop of increased Chinese pressure on the island. Taiwan has been a democratic success story since holding its first direct presidential election in 1996, the culmination of decades of struggle against authoritarian rule and martial law. The ruling Democratic Progressive Party (DPP), which champions Taiwan’s separate identity and reject…