Looking into the current standard for Synopsis Design Constraints (SDC)
from their [Technology Access
Program](https://www.synopsys.com/community/interoperability-programs/tap-in.html),
one can see that the current state of the sdc-syntax file is very
outdated as well as short in coverage of keywords.
This commit pursues to add all the missing keywords from the current
standard (Rev. 2.1).
closes: #15281
Signed-off-by: daniel-s-w <59746710+daniel-s-w@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
