Handles block start/end by calling the appropriate renderer method.
| is_start |
True to start the block, false to end it |
| block_type |
The block type |
| lang |
Language for fenced code blocks (only used when is_start is true) |
| fence_indent |
Leading indent for fenced code (e.g. " "); only used when starting FENCED_CODE_QUOTE/TILD |