Fix fold literal encoding with trailing line break (dart-archive/yaml_edit#91)

* Return null if string cannot be encoded as literal/folded

* Add test variants for trailing line breaks

Author: kekavc24

pkgs/yaml_edit/lib/src/strings.dart

@@ -96,7 +96,7 @@ String? _tryYamlEncodeSingleQuoted(String string) {
 String? _tryYamlEncodeFolded(String string, int indentSize, String lineEnding) {
   // A string that starts with space or newline followed by space can't be
   // encoded in folded mode.
-  if (string.isEmpty || string.trimLeft().length != string.length) return null;
+  if (string.isEmpty || string.trim().length != string.length) return null;
 
   if (_hasUnprintableCharacters(string)) return null;
 
@@ -164,7 +164,7 @@ String? _tryYamlEncodeFolded(String string, int indentSize, String lineEnding) {
 /// See: https://yaml.org/spec/1.2.2/#812-literal-style
 String? _tryYamlEncodeLiteral(
     String string, int indentSize, String lineEnding) {
-  if (string.isEmpty || string.trimLeft().length != string.length) return null;
+  if (string.isEmpty || string.trim().length != string.length) return null;
 
   // A string that starts with space or newline followed by space can't be
   // encoded in literal mode.

pkgs/yaml_edit/test/string_test.dart

@@ -11,6 +11,11 @@ final _testStrings = [
   'whitespace\n after line breaks',
   'whitespace\n \nbetween line breaks',
   '\n line break at the start',
+  'whitespace and line breaks at end 1\n ',
+  'whitespace and line breaks at end 2 \n \n',
+  'whitespace and line breaks at end 3 \n\n',
+  'whitespace and line breaks at end 4 \n\n ',
+  '\n\nline with multiple trailing line break \n\n\n\n\n',
   'word',
   'foo bar',
   'foo\nbar',