fix: keep // comment keys with the field below them in package.json

[dsherret]

Closes #72

With packageJson.applyConventions, a top level key starting with // ("//", "// node", ...) was treated as an unknown field and sorted to the end of the file, away from the field it was written to describe.

Such a key now travels with the field written below it, the same way a real comment does:

• several comment keys above one field keep the order they were written in
• a comment key with no field below it stays at the end
• comments and blank lines written above a comment key still move with it

Not changed here:

• comment keys inside the sections that get sorted alphabetically (engines, bin, the dependency sections) are still sorted as ordinary names
• a comment key always binds downwards, so one written after the field it describes moves with the next field instead

[dsherret]

This only did top level. Doing a new pr for nested sections.