Previously, MDDocument only split on Unix-style line endings. This adds a new function to StringView which handles LF, CR and CRLF.
This was a workaround to be able to build on case-insensitive file systems where it might get confused about <string.h> vs <String.h>. Let's just not support building that way, so String.h can have an objectively nicer name. :^)
Previously we'd implicitly convert the second StringView to a String when comparing two StringViews, which is obviously not what we wanted.
