dart: Improve indentation (#18845)

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

extensions/dart/languages/dart/indents.scm

@@ -1,18 +1,3 @@
-(class_definition
-    "class" @context
-    name: (_) @name) @item
-
-(function_signature
-    name: (_) @name) @item
-
-(getter_signature
-    "get" @context
-    name: (_) @name) @item
-
-(setter_signature
-    "set" @context
-    name: (_) @name) @item
-
-(enum_declaration
-    "enum" @context
-    name: (_) @name) @item
+(_ "[" "]" @end) @indent
+(_ "{" "}" @end) @indent
+(_ "(" ")" @end) @indent