Prefer interpolation to compose strings

Using interpolation when composing strings and values is usually
easier to write and read than concatenation.

analysis_options.yaml

@@ -26,6 +26,7 @@ linter:
     - prefer_final_locals
     - prefer_initializing_formals
     - prefer_int_literals
+    - prefer_interpolation_to_compose_strings
     - prefer_relative_imports
     - prefer_single_quotes
     - unnecessary_parenthesis

lib/models/documents/nodes/line.dart

@@ -55,7 +55,7 @@ class Line extends Container<Leaf?> {
   }
 
   @override
-  String toPlainText() => super.toPlainText() + '\n';
+  String toPlainText() => '${super.toPlainText()}\n';
 
   @override
   String toString() {

lib/utils/color.dart

@@ -119,7 +119,7 @@ Color stringToColor(String? s) {
   }
 
   var hex = s.replaceFirst('#', '');
-  hex = hex.length == 6 ? 'ff' + hex : hex;
+  hex = hex.length == 6 ? 'ff$hex' : hex;
   final val = int.parse(hex, radix: 16);
   return Color(val);
 }

lib/utils/diff_delta.dart

@@ -79,11 +79,7 @@ int getPositionDelta(Delta user, Delta actual) {
     final userOperation = userItr.next(length as int);
     final actualOperation = actualItr.next(length);
     if (userOperation.length != actualOperation.length) {
-      throw 'userOp ' +
-          userOperation.length.toString() +
-          ' does not match ' +
-          ' actualOp ' +
-          actualOperation.length.toString();
+      throw 'userOp ${userOperation.length} does not match  actualOp ${actualOperation.length}';
     }
     if (userOperation.key == actualOperation.key) {
       continue;