Adopt diff-friendly import style.

[jonathanknowles]

Summary

This PR adjusts our stylish-haskell configuration to always import each symbol on a separate line. Example:

import Some.Module.A
    ( SomeSymbolA1
    , SomeSymbolA2
    )
import Some.Module.B
    ( SomeSymbolB1
    , SomeSymbolB2
    , SomeSymbolB3
    , SomeSymbolB4
    )

Details

This change has several advantages:

1. Better cross-version compatibility: this configuration produces almost exactly the same output across versions 0.11.0.3 and 0.14.5.0 of stylish-haskell (current and latest versions respectively).
2. Better cross-tool compatibility: the output is almost identical to that produced by fourmolu. (See experimental fourmolu branch here.)
3. Improved diff-friendliness: we virtually eliminate the problem where adding or removing a single import can lead to a multi-line diff.

Adopting this style will make it easier to move to GHC 9.2 or 9.6, both of which no longer work with our current version of stylish-haskell.

Related Issues

Without this change in our configuration, upgrading stylish-haskell from version 0.11.0.3 to 0.14.5.0 exposes us to the following regression:
haskell/stylish-haskell#462

[jonathanknowles]

Note that this trick works with both older and more recent versions of stylish-haskell.

[jonathanknowles]

This style has the final ) on a separate line -- this is the same as fourmolu.

[Anviking]

lgtm