When a YAML/TOML's delimiter character sequence is included in a
frontmatter string, parser mistakes it as a delimiter. This fixes it by
checking a character right before the delimiter sequence is '\n' or it
is the beginning of the frontmatter.
Fix#1320
This rewrites `extractFrontMatterDelims` function to make it work with
an archetype file without the final EOL and adds more detailed error
messages and comments.
It also removes `matches` and `matches_quick` functions which aren't
called anywhere.
