Create a masterlist autoformatter #128
Description
As explored in loot/starfield#74, it would be good to have a way to automatically format masterlists and the prelude into a common style, but we're not satisfied by the output of existing autoformatters.
A solution might be to write our own, built using an existing YAML library that provides the ability to customise the serialisation style in the ways that we want. There are many YAML libraries available across many languages, but to narrow down the search, our requirements include the ability to round-trip anchors, aliases and comments. We probably also want to round-trip string literal line breaks (or lack of) so that the formatter doesn't break condition strings by line-breaking them in an unsupported place.
A starting point would probably be to create a YAML snippet that demonstrates what we want the output to look like for all the YAML features and structures that we use.