Prevent signature help within anonymous functions

[the-mikedavis]

Is your feature request related to a problem? Please describe.

Signature-help popping up for a call of a higher order function can be a bit noisy:

lists:map(fun(X) -> X * 2 end, Numbers)
%%         ^----------------^
%%                 ^ signature help shows all the way through here (and for `Numbers` as well)

especially for very large anonymous functions that span many lines and themselves have many function calls.

Describe the solution you'd like

I think we should prevent signature help from popping up when you're in an anonymous function. We could even fine-tune it to not show only when the anonymous function spans multiple lines.

What do you think?

Additional context

In order to implement this, we can add a clause to els_signature_help_provider:find_signature/4 that checks for an unmatched fun token like this existing clause but returns none.

Rust-analyzer now prevents signature-help on multi-line expressions: https://github.com/jonas-schievink/rust-analyzer/commit/4bb5df0ce54544df5cf89a08067f7b295b242020