Bump markdown to e3f4bd28c9e61b522f75f291d4d6cfcfeccd83ee

Changes:
```
> git log --format="%C(auto) %h %s" 5699caf..e3f4bd2
 https://dart.googlesource.com/markdown.git/+/e3f4bd2 example: update CDN asset links (#435)
 https://dart.googlesource.com/markdown.git/+/a678bfc example: add GitHub markdown CSS (#434)
 https://dart.googlesource.com/markdown.git/+/bc79c43 Merge pull request #425 from dart-lang/pq-patch-1
 https://dart.googlesource.com/markdown.git/+/4e8aa03 add pub badge
 https://dart.googlesource.com/markdown.git/+/7987e1e Remove dependency on third party package:charcode.
 https://dart.googlesource.com/markdown.git/+/90995fd Split block_parser.dart and inline_parser.dart (#422)
 https://dart.googlesource.com/markdown.git/+/8bb9062 Add trailing commas to some parameter lists to get better formatting (#420)
 https://dart.googlesource.com/markdown.git/+/1c5f2e7 Enable raw strings lint rules (#418)
 https://dart.googlesource.com/markdown.git/+/4784153 Enable use_if_null_to_convert_nulls_to_bools lint rule (#417)
 https://dart.googlesource.com/markdown.git/+/0d67e99 Enable prefer_interpolation_to_compose_strings (#416)
 https://dart.googlesource.com/markdown.git/+/5561351 Enable prefer_final_locals lint rule (#415)
 https://dart.googlesource.com/markdown.git/+/6d39147 Create DelimiterSyntax to replace TagSyntax (#407)
 https://dart.googlesource.com/markdown.git/+/4f4e899 Add caseSensitive parameter on the InlineSyntax constructor (#400)
 https://dart.googlesource.com/markdown.git/+/e16aff0 Check parser.isDone when title is null in _parseInlineBracketedLink (#394)
 https://dart.googlesource.com/markdown.git/+/3471578 Use `Uri.toFilePath()` instead of `Uri.path` for locating tests (#396)

```

Diff: https://dart.googlesource.com/markdown.git/+/5699cafa9ef004875fd7de8ae9ea00e5295e87a4~..e3f4bd28c9e61b522f75f291d4d6cfcfeccd83ee/
Change-Id: Ie04b17dfcce57fcd9e814bd8b9a09677a91136d6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/246984
Auto-Submit: Devon Carew <devoncarew@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>
Reviewed-by: Phil Quitslund <pquitslund@google.com>

DEPS

@@ -123,7 +123,7 @@ vars = {
   "linter_rev": "a8529c6692922b45bc287543b355c90d7b1286d3", # 1.24.0
   "lints_rev": "8294e5648ab49474541527e2911e72e4c5aefe55",
   "logging_rev": "f6979e3bc3b6e1847a08335b7eb6304e18986195",
-  "markdown_rev": "5699cafa9ef004875fd7de8ae9ea00e5295e87a4", # 5.0.0
+  "markdown_rev": "e3f4bd28c9e61b522f75f291d4d6cfcfeccd83ee", # 5.0.0
   "markupsafe_rev": "8f45f5cfa0009d2a70589bcda0349b8cb2b72783",
   "matcher_rev": "12cdc5fbafd666ed908359ae215d5d0306087969",
   "mime_rev": "c2c5ffd594674f32dc277521369da1557a1622d3",