Include `theme.json` in translation scan command

[strarsis]

It turns out that when using --include with wp i18n the theme.json needs to be explicitly included, too, as it would be skipped otherwise.

Edit: The latest wp i18n version (>=2.4.0) is needed for some bugfixes of otherwise blocking bugs. Until wp CLI is also newly released with the new wp i18n version bundled, the latest wp i18n version can be installed independently from the wp CLI version by using wp package install wp-cli/i18n-command:dev-main and then just using it as usual (wp i18n).