add support for passing flatten -separator to flatten in synth

[gadfort]

What are the reasons/motivation for this change?

Closes #4843

Explain how this is achieved.

Since #4839 added this flag to flatten, this change allows for the user to specify this for the synth command.

If applicable, please suggest to reviewers how they can test the change.

scratchpad -set flatten.separator "/"
synth -flatten
write_verilog test.v

[whitequark]

The scratchpad check should be in the flatten pass, not synth, shouldn't it?

[gadfort]

@whitequark sure, I can move it there (makes more sense there)

[gadfort]

Looks like the failures might be related to the switch from ubuntu22 to ubuntu24 on the github runners