Optional compiler directives

[Thomasb81]

Ieee1800-2017 is written in a way Optional compiler directives from Annex E need to be recognized in order to avoid syntax issue when they are present in a source code:

default_decay_time
default_trireg_strength
delay_mode_distributed
delay_mode_path
delay_mode_unit
delay_mode_unit
delay_mode_zero

Remain responsibility to tool implementation to honor them.

[Nic30]

OK, let's add them after 'line directive and code positions' agenda #90 is closed (It needs just 1 day, but with my nightmare life it will take some while to find that time.)