[feat] Express: Add first customization tab, metadata, to add metadata based on conditions.

[atopper]

Description

Add first customization tab, metadata, to add metadata based on conditions.

Related Issue

Motivation and Context

Many VIP projects want pages to have additional metadata block values (i.e. template, keywords, etc.). This feature allows the importer to add strings or DOM values based on conditions on the page. Also a little re-shuffling. "Free-Selecting" is an experiment of 'Express Mapping' - and that idea may change.

How Has This Been Tested?

Manually

Screenshots (if appropriate):

Rule:

    {
      "type": "metadata",
      "insertMode": "append",
      "selectors": [],
      "params": {
        "cells": {
          "template": [
            [
              "*",
              "meta[name=template]"
            ]
          ]
        }
      }
    }

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation. - Eventually
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.