This PR is a continuation of #306

It adds supports for looking for translatable strings in JSON files within the styles directory of a theme. Unlike the top-level theme.json file we don't lock any names in this case: any JSON file within the styles folder is a candidate we should consider.

How to test

Automated tests:

composer behat -- features/makepot.feature

Manual test:

• Check out this PR locally and cd to the directory.
• composer install
• Create a new directory called foo-theme that contains a styles directory.
• Within the styles directory create a my-style.json file with the following contents:

{
  "settings": {
    "color": {
      "palette": [ { "name": "My color", "slug": "slug", "color": "value" } ]
    }
  }
}

• Run vendor/bin/wp i18n make-pot foo-theme
• Verify that there's a foo-theme/foo-theme.pot file and that the "My color" string is present as well as its context "Color name". You should see something like:

msgctxt "Color name"
msgid "My color"
msgstr ""