When returning the customRenderFn from within buildCustomMarkdownCookFunction
for custom markdown engines (such as the one used by the [chat] transcripts)
we were not hoisting/unhoisting the `html_raw` tokens created by the
transcript, which meant that opts.discourse.hoisted could end up in
a state where it was null, and which caused errors and general unpleasantness.
Instead, we can just call the `cook` function that is already exported
from discourse-markdown-it, that takes care of what we did previously
plus the hoisting.
There is a companion chat commit that adds tests for this, there are
no custom markdown engine usages in core to test with.
cfe4ff8d56