Use diff-friendly import style.

This commit changes our import style configuration so that each symbol
is imported on a separate line.

This is identical to the style used by `fourmolu`.

This configuration is respected by older and newer versions of
`stylish-haskell` (specifically, versions `3.3` and `3.6.1`).

.stylish-haskell.yaml

@@ -4,7 +4,7 @@
 # https://github.com/jaspervdj/stylish-haskell/blob/master/data/stylish-haskell.yaml
 # for usage.
 
-columns: 80 # Should match .editorconfig
+columns: 1 # Force diff-friendly import style (one line per symbol).
 steps:
   - imports:
       align: none