Fix cannot update changed messages in the dictionary

[bbuhler]

Hello (:

This is a fix for #107. It clears the lookupCache for a locale when addMessages is called.

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Tests

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[bbuhler]

I just found out that you have already a related branch (https://github.com/kaisermann/svelte-i18n/tree/fix/lookup-caching).
Though my solution is a quick'n'dirty, it would unblock me with my current work.
Feel free to close this in favor of your changes.

Never mind, I confused the dates. 🤦🏼
Now I realized that your branch is from 2020 and older than the actual issue.

[bbuhler]

@kaisermann can you please merge this or give any feedback (:

[kaisermann]

Hey @bbuhler 👋 I'm not sure if clearing the whole locale cache is the best way to do this, but I don't have an alternative right now. thanks!