GDScript: add an optional message parameter to assert()

Before this patch, assert() only took the condition to assert on:

    assert(item_data)

Now, it can optionally take a string that will be printed upon failure:

    assert(item_data, item_name + " has no item data in ItemDatabase")

This makes it easier to immediately see what the issue is by being
able to write informative failure messages.

Thanks to @wiped1 for sharing their patch, upon which this is based.

Closes #17082

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