Depends on WordPress/gutenberg#37038

In WordPress/gutenberg#37038 Gutenberg is adding a new field to translate for the font sizes stored within a theme.json. This PR adds it to the i18n command.

Setup

• See https://make.wordpress.org/cli/handbook/contributions/pull-requests/#setting-up
• To run the behat tests you also need a working mysql locally:
  • Install the packages if you don't have them. In linux: sudo apt install jq mysql-server mysql-client php-mysql
  • composer install && composer behat => it didn't setup the database for me (mysql 8) so I did;
    • sudo mysql -u root
    • CREATE DATABASE IF NOT EXISTS wp_cli_test;
    • CREATE USER 'wp_cli_test'@'localhost' IDENTIFIED WITH mysql_native_password BY 'password1';
    • GRANT ALL PRIVILEGES ON wp_cli_test.* TO "wp_cli_test"@"localhost"

How to test

• Check out this PR locally and cd to the directory.
• composer install
• Edit ThemeJsonExtractor.php by substituting the line 148 $json = self::remote_get( self::THEME_JSON_SOURCE ); by $json = '';.
• Create a new directory called foo-theme and create a theme.json file within with the following contents:

{
  "version": "2",
  "settings": {
    "typography": {
      "fontSizes": [
        { "slug": "small", "size": "12px", "name": "Small", "alias": "S" }
      ]
    }
  }
}

• Run vendor/bin/wp i18n make-pot foo-theme
• Verify that there's a foo-theme/foo-theme.pot file and that it has the name and alias strings:

#: ...

#: theme.json
msgctxt "Font size name"
msgid "Small"
msgstr ""

#: theme.json
msgctxt "Short font size name"
msgid "S"
msgstr ""