Access current locale in Custom Preview Template

[delucis]

When adding a custom preview template with i18n, the template does not seem to receive a prop/data entry to indicate the current locale. This was requested in #5337, but the fix for that (#5875) only made sure the preview refreshes to receive the current locale content. For many cases, this is probably enough, but sometimes having the locale itself is helpful (i.e. the string 'en', 'zh', etc.). For example, the preview template might need to apply RTL/LTR classes or localize dates depending on the locale.

Describe the solution you'd like
A custom preview component should receive locale either as a prop or as metadata somewhere in the entry data.

Describe alternatives you've considered
Current alternatives would include trying to guess the locale from the content or duplicating data by adding a locale field to entries.