Tidy up the `validate` command function in `cmd_validate.py`

[candleindark]

As a follow up to #1743, tidy up the validate command function in cmd_validate.py so that it mainly handles arguments from the cli, setting up some environment such as logging, and delegating to other function. Particularly,

• Ensure the value for the --ignore option is a valid regex
• Move any logic for filter the validation results to _process_issues()