v0.9.0

This is a breaking change.

Redesigned properties handling

Consumers needed more control on how to emit frontmatter properties
for advanced scenarios such as renaming/remapping properties to match
the static site generator of choice. By taking this out of configuration
and instead supplying actual functions we can easily pass that control
to consumer while simultaneously removing a lot of complexity from
notion-markdown-sync itself (i.e. the notion of property-keys vs.
property-names). This simplifies the code overall.