Add [mounted]/[nomounted] and [standing]/[sitting] conditionals

Back them with ClassicAPI IsMounted() and UnitStandState("player").
mounted/nomounted are player mount state; standing = stand state 0,
sitting = any non-standing pose (its complement). Player-only, registered
in Keywords, BOOLEAN_CONDITIONALS, and STATIC_CONDITIONALS like [stealth].

Core.lua

@@ -121,6 +121,10 @@ local BOOLEAN_CONDITIONALS = {
     nocombat = true,
     stealth = true,
     nostealth = true,
+    mounted = true,
+    nomounted = true,
+    standing = true,
+    sitting = true,
     channeled = true,
     nochanneled = true,
     checkchanneled = true,