Scheme #| ... |# (R6RS-style) block comments

python-ly already supports R5RS-style block comments `#! ... !#`.

It would be nice to support R6RS-style comments, added in Guile 2 (as used by LilyPond 2.24). These are spelt as `#| ... |#`. Unlike R5RS-style comments, they are nestable.

https://www.gnu.org/software/guile/manual/html_node/Block-Comments.html