fix: make custom expr prefix completions to understand refs
Possible fix of #7929
While reviewing the postfix completion code I saw that while calling `add_custom_postfix_completions` we were doing it under the part where reference was not taken into consideration, but as we are only adding postfix completions with `Expr` scope ( [source](ba28e19b78/crates/ide-completion/src/completions/postfix.rs (L272)) )
I shifted the `add_custom_postfix_completions` call to part where references are considered
I am not sure if this is the correct fix or I am understanding the problem exactly but this small move seemed to have fixed the issue :)
12ced8f9db