Add System.System methods to TIA

[benjaminloerincz]

Add these methods of namespace System, class System to TIA:

• System.System.currentTimeMillis()
• System.System.isBatch()
• System.System.isFunctionCallback()
• System.System.isFuture()
• System.System.isQueueable()
• System.System.isRunningElasticCompute()
• System.System.isScheduled()
• System.System.now()
• System.System.process(List workItemIds, String action, String comments, String nextApprover)
• System.System.purgeOldAsyncJobs(Date dt)
• System.System.submit(List workItemIds, String comments, String nextApprover)
• System.System.today()