verifyOrAddFrontMatter regex bug

[gentlegiantJGC]

The verifyOrAddFrontMatter function in utils.ts has a bug where it will match keys that are not in the top level.
It thinks the data is there when it actually is not (because it is on a nested layer) and does not set the data as a result.

I don't know what the better solution would be here.
Perhaps using the cached version in the obsidian API to read.

I am facing a similar issue with writing the front matter for #82 where is need to overwrite that key.
In my original branch I added a function to read and write the front matter but this read the whole structure, parsed it and wrote it back. This meant that blank values got converted to null when saved which is not ideal.

[gentlegiantJGC]

Done a bit of googling and this library is able to parse, edit and write YAML while maintaining the formatting.
https://github.com/mohsen1/yawn-yaml

Demo:
https://azimi.me/yawn-yaml/demo/index.html