do not show completions for numbers

[giacomocavalieri]

This PR makes sure the completer won't return any completions when typing a number.
Right now when typing a number makes completions for functions with such number in the name pop up:

  2
// ^ [list.window_by_2]
//   [int.to_base32]
//   ...

This usually happens in IDEs like Zed that still ask for completions even if the programmer is typing in a number.
We can't control when an IDE asks for completions, but we can avoid replying with completions that are most likely not useful in this context.

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• Tests have been added for new behaviour
• The changelog has been updated for any user-facing changes

[ankddev]

I like this change!

[lpil]

Thanks!