Add an optional lens to show whether a function is exported or not

[robertoaloi]

[alanz]

FYI I did something around this some time ago, but never turned it into an action. It does say if a function is exported or not.

And needs a rebase

https://github.com/alanz/erlang_ls/tree/code-lens-export

[TheGeorge]

After some experimentation I think its best to go with a different symbol kind instead: #918