Update markdown grammar to allow better injection into unknown fenced code blocks

extensions/markdown/syntaxes/markdown.tmLanguage

@@ -2783,7 +2783,7 @@
 					<key>name</key>
 					<string>markup.fenced_code.block.markdown</string>
 					<key>begin</key>
-					<string>(^|\G)(\s*)([`~]{3,})\s*([^`~]*)?$</string>
+					<string>(^|\G)(\s*)([`~]{3,})\s*(?=([^`~]*)?$)</string>
 					<key>end</key>
 					<string>(^|\G)(\2|\s{0,3})(\3)\s*$</string>
 					<key>beginCaptures</key>

extensions/markdown/syntaxes/markdown.tmLanguage.base

@@ -416,7 +416,7 @@
 					<key>name</key>
 					<string>markup.fenced_code.block.markdown</string>
 					<key>begin</key>
-					<string>(^|\G)(\s*)([`~]{3,})\s*([^`~]*)?$</string>
+					<string>(^|\G)(\s*)([`~]{3,})\s*(?=[^`~]*)?$</string>
 					<key>end</key>
 					<string>(^|\G)(\2|\s{0,3})(\3)\s*$</string>
 					<key>beginCaptures</key>