[LANGUAGE] Warning unused var

[Felienne]

Fixes #2394 (finally!!) by checking if declared variables are actually used, also fixes #4884 that was introduced here.

Depends-On: #4880

How to test

I have added a test for the exception. To see it in action on the front-end, run code that defines but not uses a variable, and observe you get a warning but the code still runs:

Note that this PR needs #4880 to show the warning in the front-end!

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[jpelay]

I tested it and this works great! I wonder if in the future we'd want to show the user several warnings or errors at the same time? Or do you think this would overwhelm them?

[mergify]

Thank you for contributing! Your pull request is now going on the merge train (choo choo! Do not click update from main anymore, and be sure to allow changes to be pushed to your fork).

[Felienne]

I tested it and this works great! I wonder if in the future we'd want to show the user several warnings or errors at the same time? Or do you think this would overwhelm them?

Ow great question! I have to think about that a bit more!

[mergify]

Thank you for contributing! Your pull request is now going on the merge train (choo choo! Do not click update from main anymore, and be sure to allow changes to be pushed to your fork).