Removes the herb .gitignore previously removed in 7fab8be0dc, replacing
its contents with a cross-project ignore of Nix's 'result' symlink
artifact in the root .gitignore.
Moves some entries from the Vere .gitignore into the root file, as it
seems they can safely be assumed to be undesirable everywhere.
Also cleans up the root .gitignore, removing duplicate entries,
superfluous patterns, etc.
* Move extraneous stuff out of pkg/urbit/*
* s/urb/herb/g
* Removed some boilerplate for `urbit` builds.
* Build urbit tests and run them in the nix build.
