hls-cabal-plugin: No completions for `custom-setup`

With HLS 2.11, I don't get completions for `custom-setup` stanzas.

Example:

```cabal
cabal-version:      3.8
name:               cabal-with-custom-setup
version:            0.1.0.0
build-type:         Custom

custom-setup
    

library
    exposed-modules:  MyLib
    build-depends:    base
    hs-source-dirs:   src
    default-language: Haskell2010
```

We should get completions!