Support escaped closing brackets in link ref expressions

Fixes test 161 in common mark and gfm
diff --git a/pkgs/markdown/lib/src/block_parser.dart b/pkgs/markdown/lib/src/block_parser.dart
index 0c5be70..a25636c 100644
--- a/pkgs/markdown/lib/src/block_parser.dart
+++ b/pkgs/markdown/lib/src/block_parser.dart
@@ -957,7 +957,7 @@
         // Leading indentation.
         r'''^[ ]{0,3}'''
         // Reference id in brackets, and URL.
-        r'''\[([^\]]+)\]:\s*(?:<(\S+)>|(\S+))\s*'''
+        r'''\[((?:\\\]|[^\]])+)\]:\s*(?:<(\S+)>|(\S+))\s*'''
         // Title in double or single quotes, or parens.
         r'''("[^"]+"|'[^']+'|\([^)]+\)|)\s*$''',
         multiLine: true);
diff --git a/pkgs/markdown/tool/common_mark_stats.json b/pkgs/markdown/tool/common_mark_stats.json
index bc14b36..ec0674c 100644
--- a/pkgs/markdown/tool/common_mark_stats.json
+++ b/pkgs/markdown/tool/common_mark_stats.json
@@ -384,7 +384,7 @@
  "Link reference definitions": {
   "159": "strict",
   "160": "strict",
-  "161": "fail",
+  "161": "strict",
   "162": "strict",
   "163": "strict",
   "164": "strict",
diff --git a/pkgs/markdown/tool/common_mark_stats.txt b/pkgs/markdown/tool/common_mark_stats.txt
index 1d005a5..03e10f4 100644
--- a/pkgs/markdown/tool/common_mark_stats.txt
+++ b/pkgs/markdown/tool/common_mark_stats.txt
@@ -12,7 +12,7 @@
   21 of   22 –  95.5%  Images
   11 of   12 –  91.7%  Indented code blocks
    1 of    1 – 100.0%  Inlines
-  20 of   23 –  87.0%  Link reference definitions
+  21 of   23 –  91.3%  Link reference definitions
   58 of   84 –  69.0%  Links
   44 of   48 –  91.7%  List items
   18 of   24 –  75.0%  Lists
@@ -24,4 +24,4 @@
    6 of   11 –  54.5%  Tabs
    3 of    3 – 100.0%  Textual content
   17 of   19 –  89.5%  Thematic breaks
- 506 of  624 –  81.1%  TOTAL
+ 507 of  624 –  81.3%  TOTAL
diff --git a/pkgs/markdown/tool/gfm_stats.json b/pkgs/markdown/tool/gfm_stats.json
index 67a6e94..451db60 100644
--- a/pkgs/markdown/tool/gfm_stats.json
+++ b/pkgs/markdown/tool/gfm_stats.json
@@ -400,7 +400,7 @@
  "Link reference definitions": {
   "159": "strict",
   "160": "strict",
-  "161": "fail",
+  "161": "strict",
   "162": "strict",
   "163": "strict",
   "164": "strict",
diff --git a/pkgs/markdown/tool/gfm_stats.txt b/pkgs/markdown/tool/gfm_stats.txt
index 6e40ed6..7bf50f1 100644
--- a/pkgs/markdown/tool/gfm_stats.txt
+++ b/pkgs/markdown/tool/gfm_stats.txt
@@ -14,7 +14,7 @@
   21 of   22 –  95.5%  Images
   11 of   12 –  91.7%  Indented code blocks
    1 of    1 – 100.0%  Inlines
-  20 of   23 –  87.0%  Link reference definitions
+  21 of   23 –  91.3%  Link reference definitions
   58 of   84 –  69.0%  Links
   44 of   48 –  91.7%  List items
   18 of   24 –  75.0%  Lists
@@ -28,4 +28,4 @@
    6 of   11 –  54.5%  Tabs
    3 of    3 – 100.0%  Textual content
   17 of   19 –  89.5%  Thematic breaks
- 509 of  646 –  78.8%  TOTAL
+ 510 of  646 –  78.9%  TOTAL