* Fix regex for code block end detection in wikiparser
* Anchor the codeblock end fence to its own line
Close an empty code block without consuming the tiddler, while a fence
only counts when it stands alone on its line.
* Match the end fence with (^|\r?\n) so the empty block case closes at
the line start and a code line ending in ``` stays inside the block
* Add test-codeblock-parser.js covering empty blocks with language and
CRLF, delimiter newline handling, mid-line fences and a missing fence
