Related #254 #292

In WordPress/gutenberg#39322 and WordPress/gutenberg#39715 the Gutenberg plugin has added a new translatable field to theme.json files, which will be part of WordPress 6.0.

This PR does a few things:

• Adds the new field to the local i18n schema.
• Updates the code so top-level keys are picked up for translation.
• Updates the code so strings are translatable (in addition to leaf arrays, as it works now).

How to test

Automated tests:

composer behat -- features/makepot.feature

Manual test:

• Check out this PR locally and cd to the directory.
• composer install
• Comment this line in the ThemeJsonExtractor class, so the command uses the local backup file (assets/theme-i18n.json).
• Create a new directory called foo-theme that contains a theme.json file with the following contents:

{
    "title": "My style variation"
}

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

msgctxt "Style variation name"
msgid "My style variation"
msgstr ""