Add single tilde support to `StrikethroughSyntax` (dart-lang/markdown#595)

pkgs/markdown/lib/src/inline_syntaxes/strikethrough_syntax.dart

diff --git a/pkgs/markdown/lib/src/inline_syntaxes/strikethrough_syntax.dart b/pkgs/markdown/lib/src/inline_syntaxes/strikethrough_syntax.dart
index ddb0eef..ebe5e1a 100644
--- a/pkgs/markdown/lib/src/inline_syntaxes/strikethrough_syntax.dart
+++ b/pkgs/markdown/lib/src/inline_syntaxes/strikethrough_syntax.dart
@@ -13,6 +13,6 @@
           requiresDelimiterRun: true,
           allowIntraWord: true,
           startCharacter: $tilde,
-          tags: [DelimiterTag('del', 2)],
+          tags: [DelimiterTag('del', 1), DelimiterTag('del', 2)],
         );
 }

pkgs/markdown/test/gfm/strikethrough_extension.unit

diff --git a/pkgs/markdown/test/gfm/strikethrough_extension.unit b/pkgs/markdown/test/gfm/strikethrough_extension.unit
index be4d41c..0d6f75f 100644
--- a/pkgs/markdown/test/gfm/strikethrough_extension.unit
+++ b/pkgs/markdown/test/gfm/strikethrough_extension.unit
@@ -9,3 +9,11 @@
 <<<
 <p>This ~~has a</p>
 <p>new paragraph~~.</p>
+>>> single tilde
+~Hi~ there.
+<<<
+<p><del>Hi</del> there.</p>
+>>> single tilde with double tilde
+~Hi~~ there.
+<<<
+<p><del>Hi</del>~ there.</p>