Look for `theme.json` files in the `styles` directory of a theme

[oandregal]

This PR is a continuation of https://github.com/wp-cli/i18n-command/pull/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 ""

[oandregal]

cc @swissspidy this is the continuation of https://github.com/wp-cli/i18n-command/pull/306

[oandregal]

The behat tests are now throwing the following warning (hence they fail):

PHP Warning: Invalid argument supplied for foreach() in src/ThemeJsonExtractor.php on line 209

This warning also happens in main.

The reason for this warning is that the i18n schema used to translate theme.json files (see) has been updated this morning. However, the code that interprets it has not been updated it yet: https://github.com/wp-cli/i18n-command/pull/306 needs to be merged to fix that warning.

[swissspidy]

Merged #306 now