Bump markdown to 37951d151750acfae756b2e466f563c1c5119b3d

Changes:
```
> git log --format="%C(auto) %h %s" a70b93a..37951d1
 https://dart.googlesource.com/markdown.git/+/37951d1 Fix an ImageSyntax issue (#493)
 https://dart.googlesource.com/markdown.git/+/29196b7 Refactor CodeBlockSyntax (#488)
 https://dart.googlesource.com/markdown.git/+/32c52d6 benchmark: use Stopwatch over DateTime for benchmarking (#491)
 https://dart.googlesource.com/markdown.git/+/0a7a531 Refactor ATX headings (#486)
 https://dart.googlesource.com/markdown.git/+/2fab846 some cleanup (#489)
 https://dart.googlesource.com/markdown.git/+/816dd87 Refactor CodeSyntax (#487)
 https://dart.googlesource.com/markdown.git/+/a529e58 Fix HTML escape issues (#484)

```

Diff: https://dart.googlesource.com/markdown.git/+/a70b93a1a004cc1da04a3e3ea59821ef8391e0d6~..37951d151750acfae756b2e466f563c1c5119b3d/
Change-Id: I3f8419477df2280801be3cf9251e003f6c482984
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/270224
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Auto-Submit: Devon Carew <devoncarew@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

DEPS

@@ -139,7 +139,7 @@ vars = {
   "linter_rev": "8bce8b8b06c22716219207eb9d52beb93932ea44", # 1.30.0
   "lints_rev": "16bdefe1df529262a6596f79e91003ddbdbd3890",
   "logging_rev": "f322480fb9d9e83e677c08db6d09067059f7ff74",
-  "markdown_rev": "a70b93a1a004cc1da04a3e3ea59821ef8391e0d6",
+  "markdown_rev": "37951d151750acfae756b2e466f563c1c5119b3d",
   "markupsafe_rev": "8f45f5cfa0009d2a70589bcda0349b8cb2b72783",
   "matcher_rev": "9051de05db4aceed4c7a59c34a6ce24b7c05b3c6",
   "mime_rev": "c0c4c47a3d7bf696f1aa1959fb83d598baadb33c",