fix(browser): bare cat for root/empty path in fs cat (#398)

* fix(browser): bare cat for root/empty path in fs cat

Preserve no-arg `fs cat` when the session cwd is `/` by translating the
empty absolute path into anchor + bare `cat` + restore, instead of
sending `cat ''` to DOMShell.

Also treat relative empty path as bare `cat` on the current cursor.

Fixes #330

* fix(browser): read current node for empty cat paths