Always preserve expanded inherits

This changes the formatting to take into account whether inherits
are expanded onto multiple lines already, and if so, preserves that.

This is desirable because if a user writes an inherit over
multiple lines, there's most likely an intention to add more,
which would become harder when formatted.

An example of this is inheriting from `lib`, which usually starts out
with just a single value, but usually gets more later. This would've been
contracted onto a single line before this change:

    inherit (lib)
      foo
      ;

This change conforms to the standard due to this line:

> The formatter may take the input formatting into account in some cases in order to preserve multi-line syntax elements (which would otherwise have been contracted by the rules).

src/Nixfmt/Pretty.hs

@@ -129,8 +129,11 @@ instance Pretty Binder where
       pretty inherit
         <> ( if null ids
               then pretty semicolon
-              else line <> nest (sepBy (if length ids < 4 then line else hardline) ids <> line' <> pretty semicolon)
+              else line <> nest (sepBy sep ids <> line' <> pretty semicolon)
            )
+    where
+      -- Only allow a single line if it's already on a single line and has few enough elements
+      sep = if sourceLine inherit == sourceLine semicolon && length ids < 4 then line else hardline
   -- `inherit (foo) bar` statement
   pretty (Inherit inherit (Just source) ids semicolon) =
     group $
@@ -141,10 +144,13 @@ instance Pretty Binder where
                 then pretty semicolon
                 else
                   line
-                    <> sepBy (if length ids < 4 then line else hardline) ids
+                    <> sepBy sep ids
                     <> line'
                     <> pretty semicolon
           )
+    where
+      -- Only allow a single line if it's already on a single line and has few enough elements
+      sep = if sourceLine inherit == sourceLine semicolon && length ids < 4 then line else hardline
   -- `foo = bar`
   pretty (Assignment selectors assign expr semicolon) =
     group $