I18n: Make block preview title translatable

[Soean]

Description

This PR adds i18n for the Preview title of the block preview component.

[aduth]

Testing instructions: This label in Inserter > Shared:

[aduth]

👍

[aduth]

Actually, while I just merged, one potential issue here is disambiguation of the word "Preview" . Is it a noun or a verb? Good for _x context.

https://codex.wordpress.org/I18n_for_WordPress_Developers#Disambiguation_by_context

[Soean]

@aduth Thanks, maybe a context is better for some languages. In the preview component it is a noun on the button a verb. In german we have no verb for preview. So we use the noun "Vorschau" for both. I will open another PR.
Or should we wait until this context is needed for a specific language?

[aduth]

My understanding is context is good to provide anytime there's the potential for ambiguity (here, on part of speech).

In any case, here's an actual example:

https://translate.google.com/#en/es/preview

noun
  | el | avance | advance, progress, development, preview, lead, trailer
  | el | preestreno | preview
  | la | vista anticipada | preview
verb
  | preestrenar | preview
  | presentar a los críticos | preview, press view
  | ver antes que los demás | preview, press view