Link hover styling for the fields.json to go with Dev Docs

[puresanfran]

Is your feature request related to a problem? Please describe.

We currently required link hover styles in the developer documentation but don't have a default theme setting in the fields.json.

Describe the solution you'd like

It would be beneficial for theme providers in the marketplace to have a default link hover style in the boilerplate, because it is missed often in submissions.

Maybe something like:

{
            "label": "Link Hover",
            "name": "link_hover",
            "type": "color",
            "visibility": {
              "hidden_subfields": {
                "opacity": true
              }
            },
            "inherited_value": {
              "property_value_paths": {
                "color": "theme.colors.secondary_accent.color"
              }
            }
          }

Checklist

• [x ] I have read the CONTRIBUTING document.
• [x ] I have checked the issue tracker to make sure there isn’t already an open issue.