Add custom `lsp-clients-extract-signature-on-hover`

[slotThe]

It seems like https://github.com/emacs-lsp/lsp-mode/issues/4362 didn't get any traction, so I'm following what was discussed in https://github.com/emacs-lsp/lsp-haskell/issues/151 and am creating an individual PR here instead.

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Commit Summary

Add custom lsp-clients-extract-signature-on-hover

This fixes a case where a type signature would be broken up over multiple lines, and we only display the first of these.

Fixes: https://github.com/emacs-lsp/lsp-haskell/issues/151 Related: https://github.com/emacs-lsp/lsp-mode/issues/4362 Related: https://github.com/emacs-lsp/lsp-mode/pull/1740

[michaelpj]

I wonder if a PR to upstream might get more traction?

[slotThe]

This is essentially what https://github.com/emacs-lsp/lsp-mode/pull/1740 was, but that sadly also went nowhere :/