Syntax: Refactor fenced code blocks

This commit backports refactored code fences from ST4203+ to workaround
an issue, which may cause crashing ST when unbalanced fence punctuation
appears in a document.

Author: deathaxe

messages/3.6.1.md

@@ -16,5 +16,7 @@ feedback you can use [GitHub issues][issues].
 
 - embedded linter no longer complains on Bash- and Python-style comments in code blocks 
   (previously it triggered the `MD023` rule)
+- refactor syntax definitions of fenced code blocks to work around a crash 
+  caused by unbalanced fences
 
 [issues]: https://github.com/SublimeText-Markdown/MarkdownEditing/issues