Coherence of command-line formatters versus formatter plugins

As many people do, I have a gate in my CI that ensures that code passes a formatter before it is merged. That formatter (in this case `stylish-haskell`) is run as an executable.

If I also use HLS, I can format my code with `stylish-haskell` via the formatter plugin. But this will use the version of `stylish-haskell` that HLS was compiled with, not the one that I have (elsewhere) specified for use in CI.

This makes the in-editor formatting useless to me, as I will typically have to reformat with the executable version before my change will pass CI.

I can see two ways to keep things consistent.

1. HLS follows environment

Ultimately I think this would mean that HLS would have to call the executable. I understand that this is undesirable since the current model allows us to use the nicer Haskell interfaces to tools and test them easily.

2. Environment follows HLS

Perhaps we could have some way to format "as-if" using one of the HLS formatting plugins. For example:
- Ship the formatter executables with HLS, so users can call those directly.
- Expose the formatting capabilites through HLS itself as part of the CLI, e.g. `haskell-language-server --format --formatter ormolu Foo.hs`.