Serialize trigger if filter to expression to match spec. (#61)

The configuration format requires this to be serialized as "expression" not "if."

inngest/src/main/kotlin/com/inngest/InngestFunctionTriggers.kt

@@ -9,7 +9,7 @@ abstract class InngestFunctionTrigger // or interface or data class
     @JvmOverloads
     constructor(
         @Json(serializeNull = false) val event: String? = null,
-        @Json(serializeNull = false) val `if`: String? = null,
+        @Json(serializeNull = false, name = "expression") val `if`: String? = null,
         @Json(serializeNull = false) val cron: String? = null,
         // IDEA - Add timeout and re-use for cancelOn?
     )