Support JSON format and redirect to file for `report_units`

[akashlevy]

• Add a report_units -json flag to support unit reporting in JSON format
• Add support for redirect to file

[akashlevy]

@jjcherry56 let me know if you have interest in upstreaming this. If so, I can add it to the doc as well

[jjcherry56]

I can't imagine why it would be useful to print json to the console instead of a file, so it only makes sense when used with redirection. I don't know the context of why json for units is useful, but suggest defining a tcl proc sta::write_unit_json filename instead.

[akashlevy]

Yes, this is only useful when other tools need to ingest the units for post-processing. I can move this to a separate tcl proc.

[akashlevy]

Just gonna make a new PR