Previously, partial embeds (`![[note#heading]]`) would always include
the entire file into the source note. Now, such embeds will only include
the contents of the referenced heading (and any subheadings).
Links and embeds of [arbitrary blocks] remains unsupported at this time.
[arbitrary blocks]: https://publish.obsidian.md/help/How+to/Link+to+blocks
Links within an embedded note would point to other local resources
relative to the filesystem location of the note being embedded.
When a note inside a different directory would embed such a note, these
links would point to invalid locations.
Now these links are calculated relative to the top note, which ensures
these links will point to the right path.
This refactors the Context to maintain a list of all the files which
have been processed so far in a chain of embeds. This information is
then used to print a more helpful error message to users of the CLI when
RecursionLimitExceeded is returned.
