Add ability to expand paths in a paths query

[aeisenberg]

Idea would be to add parts of sub-sources/sub-sinks to the edges relation. Normally, these would not be shown, but they would be expandable on request. Ask @aschackmull for details on this idea.

[aschackmull]

Can we get this please?

Details:
In the VSCode UI, a path explanation is a list of rows. This feature is about adding at the very least indentation and preferably expandable subsections of the path. Currently the indentation level is being calculated in Java as part of the path calculation and then promptly thrown away, so the data is ready - it just needs a bit more threading through, and then it needs UI support.
This would make it much easier to inspect path explanations as one would be able to see the structure and not just the flattened list of rows.

[aschackmull]

See also https://github.com/github/codeql-team/issues/74