Originally discovered in #120 and cherry-picked from #125.

The previous parser, `remark-parse` v7, included a transitive dependency
on an npm package with a security vulnerability. Newer versions of
`remark-parse` are wrappers around a new underlying parser,
`mdast-util-from-markdown`, so we can use that directly.

The previous parser also failed to preserve `\r\n` line endings,
replacing them all with `\n`. The new parser correctly preserves `\r\n`
line endings, finally providing a fix for the failing test case I
cherry-picked in the previous commit. The improved behavior also
uncovered an incorrect line ending test assertion that this commit
corrects.

While this change is in theory fully compatible, containing just bug
fixes, I'm tagging it `Update:` in case there are compatibility changes
in the new parser. This is consistent with #175, which upgraded
`remark-parse` v5 to v7 in a semver-minor `Update:` change.