Add "key not found" callback

[theolundqvist]

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Clear and concise description of the problem

Adding keys manually is annoying and extracting them with Spittal/vue-i18n-extract is not always possible.

Suggested solution

i18next has a solution where a callback can be added to send the missing key to a server or even add it directly to the file during development mode.

Alternative

No response

Additional context

No response

Validations

• [X] Read the Contributing Guidelines
• [X] Read the Documentation
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.