fix: autocomplete constants inside format strings
Hi! This PR adds autocompletion for constants (including statics) inside format strings and closes#16608.
I'm not sure about adding the `constants` field to the `CompletionContext`. It kinda makes sense, since it's in line with the `locals` field, and this way everything looks a bit cleaner, but at the same time does it really need to be there?
Anyway, let me know if anything should/can be changed. :)
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