Manage YAML comment indentation

[cognifloyd]

This is probably not perfect but it indents more of the comment lines as prettier does.
Ideally, we need to peek at the next object to determine how to format the comment, but the Emitter does not have that information.
Hopefully a future release of ruamel.yaml will move more of the comment lines from "post" comments to "pre" comments so that the Emitter does not need to guess about what comes next.
That involves changing the parsing layer to split up the comments somehow, but it is too big a change for us to implement by extending ruamel.yaml.

This PR also fixes an issue where empty flow maps were written as { } instead of {}.

• Don't dedent full line comments
• Handle full line comments more like prettier
• no space in empty flow maps and better full line comment indentation