Support `cabal-mode`

[dschrempf]

There is a cabal-mode in the making [1-3]. We should support it, probably when it lands in MELPA.

I think this only affects the major-modes list in the make-lsp-client call.

[1] cabal-mode: https://github.com/webdevred/cabal-mode
[2] MELPA PR: melpa/melpa#9656
[3] Upstream discussion (haskell-mode): haskell/haskell-mode#1879