Add support deepl pro api

[vladkryv]

No description provided.

[FilipePS]

Adding a PRO API should not replace the FREE API.

[vladkryv]

Adding a PRO API should not replace the FREE API.

The only difference is in the endpoint: all free API keys end with :fx, which can be used to determine which one to call. The main changes are in two lines:

1. apiKey.endsWith(":fx") ? "https://api-free.deepl.com/v2/translate" : "https://api.deepl.com/v2/translate"
2. apiKey.endsWith(":fx") ? "https://api-free.deepl.com/v2/usage" : "https://api.deepl.com/v2/usage"

The free API has not been removed; only the free prefix in the names was removed because, after the change, both free and pro will work. (But yes, this should have been done in separate commits.)

[vladkryv]

Also forgot to mention: with the current changes, users' previously entered deepl_freeapi key will not be preserved after the update, as it is now stored as deepl_api in the configuration. It might be worth adding a migration/renaming in the code that runs after the update and during settings import, but I'm not sure how to implement it properly.