Add checks for validating translations in {% schema %}

[macournoyer]

{% schema %} contains JSON that can contain locales. Similar to the ones under locales/:

{% schema %}
  {
    "locales": {
      "en": {
        "title": "Welcome",
        "missing": "Product"
      },
      "fr": {
        "title": "Bienvenue",
        "extra": "Extra"
      }
    }
  }
{% endschema %}

This new check MatchingSchemaTranslations, applies the same check to "locales" as MatchingTranslations does to locales/*.json. I refactored the common code under LocaleDiff.

Also, an undocumented feature of the schema tag is that all keys that end up in the UI can be localized too:

{% schema %}
{
  "name": {
    "cs": "Seznam kolekcí",
    "da": "Kollektionsliste",
    "de": "Kategorieliste",
    "en": "Collection list",
    "es": "Lista de colecciones",
  },
  "settings": [
    {
      "type": "text",
      "id": "title",
      "label": {
        "cs": "Nadpis",
        "da": "Overskrift",
        "de": "Überschrift",
        "en": "Heading",
        "es": "Encabezado",
    ...
}
{% endschema %}

This new check, makes sure all keys have defined all the known locales used in the schema.

Not perfect, but it does match the errors of existing checks on project-64k (yarn build --checkTargetLocales):

sections/collection-list.liquid:67: suggestion: MatchingSchemaTranslations: settings.swipe_on_mobile.label missing translations for cs, da, de, es, fi, fr, it, ja, ko, nb, nl, pl, pt-BR, pt-PT, sv, th, tr, vi, zh-CN, zh-TW.
	{% schema %}
	   ^^^^^^^

sections/featured-collection.liquid:105: suggestion: MatchingSchemaTranslations: settings.swipe_on_mobile.label missing translations for cs, da, de, es, fi, fr, it, ja, ko, nb, nl, pl, pt-BR, pt-PT, sv, th, tr, vi, zh-CN, zh-TW.
	{% schema %}
	   ^^^^^^^

sections/main-collection.liquid:98: suggestion: MatchingSchemaTranslations: settings.center_align_text.label missing translations for cs, da, de, es, fi, fr, it, ja, ko, nb, nl, pl, pt-BR, pt-PT, sv, th, tr, vi, zh-CN, zh-TW.
	{% schema %}
	   ^^^^^^^

Fix #68

[jbourassa]

Neat! 👍

[jbourassa]

👌 good renaming, much better than the previous ones!

[jbourassa]

Oh, we might even be able to increment that :1 with the line number from the {% schema %} tag (one day, not now).

[macournoyer]

Yeah! I created a ticket to do this: #71. That would be awesome!