Add Modifier categories.

These are used when a normal syntactic category is attached as a modifier instead of as a statement, e.g.:

```ruby
if foo then bar end
```

vs.

```ruby
bar if foo
```

src/Category.hs

@@ -222,6 +222,8 @@ data Category
   | FieldDeclarations
   -- | A Go rune literal.
   | RuneLiteral
+  -- | A modifier version of another Category, e.g. Ruby’s trailing @if@, @while@, etc. terms, whose subterms are swapped relative to regular @if@, @while@, etc. terms.
+  | Modifier Category
   deriving (Eq, Generic, Ord, Show)
 
 -- Instances
@@ -347,3 +349,4 @@ instance Listable Category where
     \/ cons0 FieldDeclarations
     \/ cons0 RuneLiteral
     \/ cons1 (Other . unListableText)
+    \/ cons1 Modifier

src/DiffSummary.hs

@@ -477,6 +477,7 @@ instance HasCategory Category where
     C.QualifiedIdentifier -> "qualified identifier"
     C.FieldDeclarations -> "field declarations"
     C.RuneLiteral -> "rune literal"
+    C.Modifier c -> toCategoryName c <> " modifier"
 
 instance HasField fields Category => HasCategory (SyntaxTerm leaf fields) where
   toCategoryName = toCategoryName . category . extract

src/Renderer/Split.hs

@@ -145,6 +145,7 @@ styleName category = "category-" <> case category of
   C.IndexExpression -> "index_expression"
   C.FieldDeclarations -> "field_declarations"
   C.RuneLiteral -> "rune_literal"
+  C.Modifier c -> styleName c <> "_modifier"
 
 -- | Pick the class name for a split patch.
 splitPatchToClassName :: SplitPatch a -> AttributeValue