[mv3] Add support for custom scriptlet filters

At commit time, only Chromium supports custom scriptlet filters, and
this also requires that the user enables the "Allow User Scripts"
setting in the extension details.

Custom scriptlet filters can be entered through the "Import/Export"
field in the "Custom filters" pane in the dashboard. Once a custom
scriptlet filter i spresent in the list of filters, it can be freely
edited.

Custom scriptlet filters are enforced in basic and higher filtering
modes -- just as is the case with custom cosmetic filters.

This new capability is mostly of interest to filter list authors.