Make dump_stackup_to_json.tcl write out the power_strap_width_table

[harrisonliew]

This isn't a database object in Innovus (i.e. a get_db layer:<layer> .* doesn't return anything that is the width table), so it's not trivial.

The closest thing is getting the table of width_at_leasts and appending the max_width to the end, but this may not be the case in all technologies. Perhaps this will just have to be done manually for new technologies.