Add folgezettel queries

[srid]

Adds a couple of queries to list notes by folgezettel relationship:

• children:. - list folgezettel children
• parents:. - list folgezettel parents

This is useful on its own (as a query), but also serves as a first step towards #473

• Initial impl
• Support children:foo to list children of [[foo]] note
  • With children:. listing children of current note. Much like path:./*
• Docs