Adopt cabal-add?

**Describe the feature request**
People often ask for a programmatic way to manipulate parts of the cabal file, especially w.r.t dependencies. A recent tool, [cabal-add](https://github.com/Bodigrim/cabal-add), delivers on the particular task. Should it be integrated into `cabal` proper?

**Additional context**
`cabal-add` (a tool adding entries in `build-depends` without ruining formatting in a cabal file) is being integrated into HLS: https://github.com/Bodigrim/cabal-add/issues/7 This confirms my feeling that there's a user base for this feature, however partial this feature is.

I tried to read the implementation (it's reasonably small and clean). It juggles between `[Field Position]` and `GenericPackageDescription` like no tomorrow, starting from:
https://github.com/Bodigrim/cabal-add/blob/267eb6802477076b64568bff774aa7833bcbfa7b/src/Distribution/Client/Add.hs#L202-L210

... as is expected in the absence of a holistic solution for exact parsing (cf. https://github.com/haskell/cabal/issues/7544).

The core of it is three "algorithms" to solve the task (add a dependency) that spread across the spectrum: accuracy --- robustness (e.g. the first algorithm preserves formatting the best but is more fragile than the others)

- https://github.com/Bodigrim/cabal-add/blob/267eb6802477076b64568bff774aa7833bcbfa7b/src/Distribution/Client/Add.hs#L403-L407
- https://github.com/Bodigrim/cabal-add/blob/267eb6802477076b64568bff774aa7833bcbfa7b/src/Distribution/Client/Add.hs#L450-L453
- https://github.com/Bodigrim/cabal-add/blob/267eb6802477076b64568bff774aa7833bcbfa7b/src/Distribution/Client/Add.hs#L472-L475

Given that all of them are `Maybe`, I assume, the tool doesn't guarantee success...

Maybe we should wait and see how the HLS feature fares and what its users think?

Start by reading cabal-add's Distribution/Client/Add.hs, especially the cited sections around lines 202-210, 403-407, 450-453, and 472-475. Review the linked HLS integration and the exact-parsing discussion in issue 7544, then determine whether the feature has a sufficiently defined scope and robustness expectations for cabal proper; done means an agreed integration direction or explicit decision not to adopt it.

Written by the indexing model from the issue text.